wlcore: tx_flush - optimize flow and force Tx during the flush

Force Tx during the flush if there are packets pending in the driver.
This actually solves a bug where we would get called from the mac80211
wq context, which would prevent tx_work from getting queued, even when
the mutex is unlocked.

Don't stop the queues needlessly if there's nothing to flush. Use a
larger delay when sleeping to give the driver a chance to flush and
avoid cpu busy looping. Re-arrange the loop so the last iteration is
not wasted.

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