radio: iris: Fix the possibility of NULL pointer access

- Fix the issue with null pointer access.
- When SSR triggers, SMD channel closes and
  FM driver will unregister the device.
- While unregister device, free the memory allocation
  of the device pointer.
- Added a NULL check, before accessing device.

Change-Id: I7233299d07494124a8c3628847919421970d9ceb
CRs-Fixed: 838183
Signed-off-by: Venkateshwarlu Domakonda <vdomak@codeaurora.org>