wl12xx: implement the tx_frames_pending mac80211 callback

Frames are considered pending when they reside in the driver TX queue or
already queued in the FW.

This notion of "pending" is appropriate for power save considerations in
STA mode, but not necessarily in other modes (for instance P2P-GO).

[Fixed a sparse warning about missing "static" in a function
declaration -- Luca]

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