[Ada] Fix serial port control setting on GNU/Linux

[Pierre-Marie de Rodat]

This fixes an issue when setting the control flags of the serial
communication port. The c_cflag termios field should not be set with the
baud rate. The speed of the communication is set using the c_ispeed and
c_ospeed fields. Setting the baud rate into the c_cflag has unexpected
results as it will wrongly set some bits used to control other aspects
of the communication channel (bits, parity...).

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* libgnat/g-sercom__linux.adb (Set): Fix control flags of the
	serial port setting.
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