mxser: remove xmit_cnt < 0 tests

mxser_port::xmit_cnt is unsigned, it cannot be negative. Therefore
remove all the xmit_cnt < 0 tests.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-61-jslaby@suse.cz


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