fix(serial): warning Warray-bounds

[fpistm]

when USB CDC enabled and generic Serial

In member function 'void HardwareSerial::setRx(uint32_t)',
    inlined from 'HardwareSerial::HardwareSerial(void*, HalfDuplexMode_t)' at C:\Users\<user>\AppData\Local\Arduino15\packages\STMicroelectronics\hardware\stm32\2.5.0-dev\cores\arduino\HardwareSerial.cpp:137:10:
C:\Users\<user>\AppData\Local\Arduino15\packages\STMicroelectronics\hardware\stm32\2.5.0-dev\cores\arduino\HardwareSerial.cpp:573:18: warning: array subscript 'HardwareSerial[0]' is partly outside array bounds of 'USBSerial [1]' [-Warray-bounds]
  573 |   _serial.pin_rx = digitalPinToPinName(_rx);
In file included from C:\Users\<user>\AppData\Local\Arduino15\packages\STMicroelectronics\hardware\stm32\2.5.0-dev\cores\arduino\WSerial.h:6,
                 from C:\Users\<user>\AppData\Local\Arduino15\packages\STMicroelectronics\hardware\stm32\2.5.0-dev\cores\arduino\wiring.h:48,
                 from C:\Users\<user>\AppData\Local\Arduino15\packages\STMicroelectronics\hardware\stm32\2.5.0-dev\cores\arduino\Arduino.h:36,
                 from C:\Users\<user>\AppData\Local\Arduino15\packages\STMicroelectronics\hardware\stm32\2.5.0-dev\cores\arduino\HardwareSerial.cpp:26:
C:\Users\<user>\AppData\Local\Arduino15\packages\STMicroelectronics\hardware\stm32\2.5.0-dev\cores\arduino\USBSerial.h: In constructor 'HardwareSerial::HardwareSerial(void*, HalfDuplexMode_t)':
C:\Users\<user>\AppData\Local\Arduino15\packages\STMicroelectronics\hardware\stm32\2.5.0-dev\cores\arduino\USBSerial.h:72:18: note: object 'SerialUSB' of size 16
   72 | extern USBSerial SerialUSB;