Fix avr109 extended address

[MCUdude]

Fix pulled from issue #454. Makes it possible to write to flash addresses above the 128kiB mark.

Resolves issue #360 and issue #454.

Without this PR:

$ ./avrdude -cavr109 -p atxmega256a3bu -b 115200 -P /dev/cu.usbserial-1410 -e -Uflash:w:../../avrdude-terminal/src/0x55_256kib.hex:i

Connecting to programmer: .
Found programmer: Id = "XBoot++"; type = S
    Software Version = 1.7; No Hardware Version given.
Programmer supports auto addr increment.
Programmer supports buffered memory access with buffersize=512 bytes.

Programmer supports the following devices:
    Device code: 0x7b

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.00s

avrdude: Device signature = 0x1e9843 (probably x256a3bu)
avrdude: erasing chip
avrdude: reading input file ../../avrdude-terminal/src/0x55_256kib.hex for flash
avrdude: writing 262143 bytes flash ...

Writing | ################################################## | 100% 31.81s

avrdude: 262143 bytes of flash written
avrdude: verifying flash memory against ../../avrdude-terminal/src/0x55_256kib.hex

Reading | ################################################## | 100% 25.64s

avrdude: verification error, first mismatch at byte 0x1ffff
         0xff != 0x55
avrdude: verification error; content mismatch

avrdude done.  Thank you.

With this PR:

$ ./avrdude -cavr109 -p atxmega256a3bu -b 115200 -P /dev/cu.usbserial-1410 -e -Uflash:w:../../avrdude-terminal/src/0x55_256kib.hex:i

Connecting to programmer: .
Found programmer: Id = "XBoot++"; type = S
    Software Version = 1.7; No Hardware Version given.
Programmer supports auto addr increment.
Programmer supports buffered memory access with buffersize=512 bytes.

Programmer supports the following devices:
    Device code: 0x7b

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.00s

avrdude: Device signature = 0x1e9843 (probably x256a3bu)
avrdude: erasing chip
avrdude: reading input file ../../avrdude-terminal/src/0x55_256kib.hex for flash
avrdude: writing 262143 bytes flash ...

Writing | ################################################## | 100% 31.83s

avrdude: 262143 bytes of flash written
avrdude: verifying flash memory against ../../avrdude-terminal/src/0x55_256kib.hex

Reading | ################################################## | 100% 25.68s

avrdude: 262143 bytes of flash verified

avrdude done.  Thank you.

[mcuee]

@stefanrueger

Probably you can take a look at this one. Thanks.

[stefanrueger]

Looks good! The code makes sense and it has been proven correct through testing.

[mcuee]

Great. Then this can be merged.