wil6210: Tx performance monitoring

For performance monitoring, trace time intervals when Tx vring
is idle/not idle. Use CPU cycle counter for this, because jiffies is
too rough, and other precise time measurement methods involve
overhead while get_cycles() should be fast.
This used to provide some estimation for percentage when Tx vring
was idle, i.e. when hardware is under-utilized.
Estimation is not precise because of many reasons - CPU frequency scaling,
grt_cycles() may be per core etc. But still, it is good estimation

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>