iwlwifi: nvm: honour VHT enable flag

Some devices don't support VHT (802.11ac) and this is encoded
in the data stored in the NVM. Read the flag and use it to set
up the VHT capabilities accordingly.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>