tty: Fix lookahead_buf crash with serdev

tty: Fix lookahead_buf crash with serdev

Do not follow a NULL pointer if the tty_port_client_operations does not
implement the ->lookahead_buf() callback, which is the case with
serdev's ttyport.

Reported-by: Hans de Goede <hdegoede@redhat.com>
Fixes: 6bb6fa6908ebd3 ("tty: Implement lookahead to process XON/XOFF timely")
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Link: https://lore.kernel.org/r/20220818115026.2237893-1-vincent.whitchurch@axis.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>