mxser: remove tty parameter from mxser_receive_chars_new()

After the previous change (no plays with of tty->receive_room), the tty
parameter is unused.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211118073125.12283-16-jslaby@suse.cz


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