iwlwifi: mvm: use firmware station lookup, combine code

In most cases, the firmware will already match the station that
we received a given frame from and tell us the station ID in the
RX status, so we can look up the station from that. This lets us
skip the (more expensive) hash table lookup in mac80211.

Also change the fallback case (no station info from the firmware)
to not attempt to look up a multicast source address.

While at it, also combine all the code using the station into a
single if block.

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