radio: iris: Add NULL pointer check

Dereferencing a NULL pointer may cause kernel crash,
Check for NULL pointer before dereferencing

Conflicts:
        drivers/media/radio/radio-iris.c

Change-Id: Ic3368d619d23c76702d95e0f69e41394e423a296
CRs-Fixed: 548404
Signed-off-by: Ayaz Ahmad <aahmad@codeaurora.org>