iwlwifi: mvm: prevent the NIC to be powered at driver load time.

Some NICs aren't allowed to be powered up at driver load time.
Fix it, and move the external NVM loading from driver load time to
driver up time (parsing the external nvm file remains at driver load time).

Signed-off-by: Eytan Lifshitz <eytan.lifshitz@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>