[media] lirc_zilog: Fix NULL comparisons style

Fix all checkpatch reported issues for "CHECK: Comparison to NULL could
be written...".

Do these comparisons using the recommended coding style and consistent
with other similar cases in the file, which already used the recommended
way.

Signed-off-by: Ricardo Silva <rjpdasilva@gmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>