V4L/DVB: ir-core: prepare to add more operations for ir decoders

Some decoders and a lirc_dev interface may need some other operations to work.
For example: IR device register/unregister and ir_keydown events may need to
be tracked.

As some operations can occur in interrupt time, and a lock is needed to prevent
un-registering a decode while decoding a key, the lock needed to be convert
into a spin lock.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>