Check return value of ioctl TIOCGSERIAL on Linux

[Arth-ur]

In the fix for issue #153, the return value of the ioctl TIOCGSERIAL should be check to ensure that the serInfo variable has correctly been set before using it for TIOCSSERIAL.
Since Linux kernel 3.19, the TIOCSSERIAL ioctl prints a warning message when called (tty_warn_deprecated_flags). This behaviour will change in the next kernel version (4.20) but in the mean time this warning is spammed to the users.

In this PR, I check the return value of the TIOCGSERIAL ioctl which generates no warning. A return value not equal to 0 means that TIOCGSERIAL is not handled and TIOCSSERIAL is either not handled or generates a warning. When the return value is not 0, I skip the TIOCSSERIAL ioctl to avoid the warning.