iwlwifi: mvm: use iwl_mvm_sta_from_mac80211() consistently

A number of places (still) use a direct operation, use
iwl_mvm_sta_from_mac80211() consistently. In one place
also move it into the variable initializer.

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