mxser: extract mxser_ioctl_op_mode

This ioctl code sets RS232, RS422 or RS485 mode. Move the code to a
separate function (mxser_ioctl_op_mode) with a note that these
mxser-special ioctls should be likely replaced eventually by TIOCGRS485
and TIOCSRS485.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-39-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>