mxser: remove unnesesary NULL check

mxser_transmit_chars(tty, port) is called only from mxser_interrupt().
NULL check is performed in mxser_interrupt() so it is redundant here.

Signed-off-by: Kulikov Vasiliy <segooon@gmail.com>
Acked-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>