[media] lirc_zilog: Convert the instance open count to an atomic_t

The open count is simply used for deciding if the Rx polling thread
needs to poll the IR chip for userspace.  Simplify the manipulation
of the open count by using an atomic_t and not requiring a lock
The polling thread errantly didn't try to take the lock anyway.

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>