iwlwifi: Fix memory leak in iwl_req_fw_callback()

In this routine, kzalloc allocates a memory block. This allocation is
freed in the error paths, but not in the normal exit, thus the allocation
is leaked.

The kmemleak facility was used to find the leak.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Johannes Berg <johannes.berg@intel.com>
Cc: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Cc: Intel Linux Wireless <ilw@linux.intel.com>