mac80211: allow configure_filter callback to sleep

Over time, a whole bunch of drivers have come up
with their own scheme to delay the configure_filter
operation to a workqueue. To be able to simplify
things, allow configure_filter to sleep, and add
a new prepare_multicast callback that drivers that
need the multicast address list implement. This new
callback must be atomic, but most drivers either
don't care or just calculate a hash which can be
done atomically and then uploaded to the hardware
non-atomically.

A cursory look suggests that at76c50x-usb, ar9170,
mwl8k (which is actually very broken now), rt2x00,
wl1251, wl1271 and zd1211 should make use of this
new capability.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>