wlcore/wl12xx: add hw op for getting rx packet data length

There is a difference in the way chip families report the length of data
in a single Rx packet. Abstract this into a HW op. Refactor the Rx data
handling function to allocate the correct size for the data, and avoid
trimming the skb.

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