wil6210: add support for link statistics

Driver can request FW to report link statistics using
WMI_LINK_STATS_CMDID.
FW will report statistics with WMI_LINK_STATS_EVENTID.
Two categories of statistics defined: basic and global.

New "link_stats" debugfs is used for requesting basic statistics
report (write) and for reading the basic statistics (read).
"link_stats_global" debugfs is used for requesting and reading the
global statistics.

Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>