[media] [staging] lirc_serial: allocate irq at init time

There's really no good reason not to just grab the desired IRQ at driver
init time, instead of every time the lirc device node is accessed. This
also improves the speed and reliability with which a serial transmitter
can operate, as back-to-back transmission attempts (i.e., channel change
to a multi-digit channel) don't have to spend time acquiring and then
releasing the IRQ for every digit, sometimes multiple times, if lircd
has been told to use the min_repeat parameter.

CC: devel@driverdev.osuosl.org
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>