[media] lirc_zilog: Remove unneeded rx->buf_lock

Remove the rx->buf_lock that protected the rx->buf lirc_buffer.  The
underlying operations on the objects within the lirc_buffer are already
protected by spinlocks, or the objects are constant (e.g. chunk_size).

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>