wl1271: Fix TX queue low watermark handling

wl1271: Fix TX queue low watermark handling

The number of entries in the TX queue is compared to the low watermark
value each time TX completion interrupts are handled.
However, the fact that a TX completion arrived does not necessarily mean
there are any less skbs in the TX queue.

In addition, a TX completion interrupt does not necessarily mean that there
are any new available TX blocks. Thus, queuing TX work when the low
watermark is reached might not be needed.

Fix this by moving the low watermark handling to the TX work function,
and avoid queuing TX work in this case.

Signed-off-by: Ido Yariv <ido@wizery.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>