[media] saa7134: simplify timer activation

This simplies the code and resolves a possible race condition between
ir_raw_decode_timer_end() and saa7134_raw_decode_irq().

If the interrupt handler is called after ir_raw_decode_timer_end()
calls ir_raw_event_handle() but before clearing ir->active, then the
timer won't be rearmed.

Compile tested only.

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