wl12xx: set the skbuff priority for dummy packets

The firmware requires dummy packets to be sent using TID 7
(WL1271_TID_MGMT).  Instead of hardcoding it in the tx_fill_hdr()
function, set it when creating the packet itself.

This requires Eliad's fix to set the actual TID in the TX descriptor.

Cc: Ido Yariv <ido@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>