iwmc3200wifi: Check for cmd pointer before dereferencing it

The wifi_if_wrapper notification handling code uses a cmd pointer without
checking if it's valid or not. We're dereferencing it because we assume that
we only get to that point if there was a pending command for us. That's not
always true, so we'd better check.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>