wil6210: interrupt statistics

Track number of interrupts and Tx/Rx packets;
expose through debugfs 'info'. Reset upon read.
Used to analyse effectivness of interrupt coalescing and NAPI.
Read twice with some interval like
cat info > /dev/null; sleep 1; cat info

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