wl18xx: limit Tx for the AP single-STA-in-PSM case

Treat a single connected STA in PSM as a slow link and regulate Tx speed
according to slow link priority/stop thresholds.
This allows us to avoid flooding the FW, while delivering decent
throughput to a peer in forced-PSM.

[Small simplification of the if statements -- Luca]

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