mxser: remove cnt from mxser_receive_chars

After the previous ioctls removal, cnt is needed only in
mxser_receive_chars_old now. So remove it from mxser_receive_chars and
mxser_receive_chars_new and account only in mxser_receive_chars_old.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-32-jslaby@suse.cz


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