mxser: remove pointless xmit_buf checks

xmit_buf is supposed to exist in all these functions. I.e. from
tty_port_operations::activate() to ::shutdown(). So remove these checks.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211118073125.12283-7-jslaby@suse.cz


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