wil6210: interrupt moderation

Use hardware capabilities to limit IRQ generation to about 15 per msec
It corresponds to about 7 packets/IRQ when running iperf with default
parameters at 1.3Gbps

Do not enable this feature in the sniffer (monitor)  mode, because
interrupt moderation cause timestamp accuracy deterioration.
For the sniffer flow, it is important to get precise timestamp.

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