iwlwifi: mvm: advertise NETIF_F_SG

If the transport supports it, advertise NETIF_F_SG to mac80211 to
be able to use frag SKBs. This will already improve performance by
allowing software GSO to be used.

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