Maybe you just set a baud rate nearest to the one from the table in
case of BOTHER?

This is broken. BOTHER can be set with a perfectly valid baud rate that
could equally be represented by B9600 say.

If you are stuck with limited ranges then

	switch(baud) {
	case 9600:
	case 4800:

etc

and don't worry about BOTHER, it's entirely transparent to you. The core
kernel code will provide you with a baud rate number, the re-encoder will
always do the right thing.

A driver should never care about BOTHER or any of the baud bits in the
termios structure directly.

Alan