[media] serial_ir: generate timeout

No timeout is generated by serial_ir since the port only generates
interrupts on edges. Some IR protocols like rc6 and rc5 need a trailing
space or timeout so they know there are no more bits coming.

Without it, the current key will only be reported once some more IR
occurs.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>