[media] staging: media: lirc: replace NULL comparisons with !var

A NULL comparison can be written as if (var) or if (!var).
Reported by checkpatch.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>