wlcore/wl18xx: add hw op for setting Tx HW checksum

Some chip families are capable of checksumming certain classes of Tx
packets in HW. Indicate this fact in the netdev features and perform the
HW checksum by protocol type for the 18xx family.

Fix the location of the skb network header when we move it so we can
rely on it when setting the checksum.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>