wlcore/wl18xx: add hw op for Rx HW checksum

Some chip families can checksum certain classes of Rx packets in FW.
Implement the Rx-checksum feature as a HW-op. For the 18xx chip-family,
set Rx-checsum according to indication from FW.

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