mac80211: add NAPI support back

NAPI was originally added to mac80211 a long time ago (by John in
commit 4e6cbfd0 in July 2010), but then removed years later
(by Stanislaw in commit 30c97120 in February 2013). No driver
ever used it, so that was fine.

Now I'm adding support for NAPI to our driver, so add some code
to mac80211 again  to support NAPI. John was originally wrapping
some (but not nearly all NAPI-related functions), but that doesn't
scale very well with the number of functions that are there, some
of which are even only inlines. Thus, instead of doing that, let
the drivers manage the NAPI struct, except for napi_add() which is
needed so mac80211 knows how to call napi_gro_receive().

Also remove some no longer needed definitions that were left when
NAPI support was removed.

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