wl18xx: use link priority thresholds as valid upper limits

The number of allocated packets can attain the value of the threshold.
The FW sets the suspend threshold to the number of Tx descriptors.
Therefore the current situation always kept the last descriptor unused.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
wlcore: debugfs: allow changing tx pacing settings

Signed-off-by: Arik Nemtsov <arik@wizery.com>