iwlwifi: mvm: don't return uninitialized value in get_survey()

If ucode_loaded isn't true the function returns the 'ret' variable
without having assigned a value properly. Fix that.

Reported-by: Haim Dreyfuss <haim.dreyfuss@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>