mxser: make xmit ring buffer variables unsigned

Make the xmit ring buffer pointers and counter (xmit_head, xmit_tail,
xmit_cnt) unsigned. Now, the assumptions for the compiler are clear --
they cannot be negative.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-20-jslaby@suse.cz


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