mac80211: add NETIF_F_RXCSUM to features white list

NETIF_F_RXCSUM is not in the white list, though some
drivers may want to set it in order to enable seeing the
actual RX checksum status in ethtool.

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