mac80211: allow low-level drivers to set netdev feature bits

Low level drivers can now set certain netdev feature bits in
netdev_features member of the ieee80211_hw struct. These will be
propagated to every netdev created from this HW.

The white-listed features currently include only ones related to HW
checksumming.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>