iwlwifi: mvm: make iwl_mvm_update_mcc() easier to follow

Some static checkers (e.g. smatch) complain about the logic, saying that
resp_cp might be leaked. Clearly that isn't true, but making the logic
easier to follow does not result in any significant code changes and makes
the code more readable by moving the NULL check closer to its source.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>