iwlwifi: mvm: add iwl_mvm_sta_from_mac80211

This is just a helper function to go from the mac80211
station struct to our internal one, to later allow us
to avoid temporary 'mvmsta' variables.

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