[media] lirc_zilog: Don't acquire the rx->buf_lock in the poll() function

There is no need to take the rx->buf_lock in the the poll() function
as all the underling calls made on objects in the rx->buf lirc_buffer object
are protected by spinlocks.

Corrected a bad error return value in poll(): return POLLERR instead
of -ENODEV.

Added some comments to poll() for when, in the future, I forget what
poll() and poll_wait() are supposed to do.

[Jarod: minor debug spew fix]

Signed-off-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>