wlcore/wl12xx: add hw op for setting frame length in tx_hw_desc

Each chip family indicates the length of a frame to the HW differently.
This includes different padding, alignment and other fields in the HW Tx
descriptor.

Put all wl12xx specific code in a hw op.

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