iwlwifi: don't double free a pointer if no FW was found

In the very unlikely case in which no firmware could be,
found. the same pointer was freed twice. Fix that.

Fixes: 490fefeb ("iwlwifi: define the .ucode file format for debug")
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>