wlcore/wl12xx: split Tx completion to immediate/delayed

One chip family employs immediate Tx completion, where knowledge of
completed packets is given as part of the FW status. Another is only
notified of Tx completion via the FW status, and has to read the
completion status of the packets from a different location.

Implement the wl12xx tx completion as a delayed Tx completion.

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