wil6210: Change of threshold for tx vring idleness measurement

Change threshold to be variable debugfs entry from hard-coded 0.
Default threshold value is 16 descriptors because HW is capable
of fetching up to 16 descriptors at once.

Signed-off-by: Vladimir Shulman <qca_shulmanv@qca.qualcomm.com>
Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>