ath9k: Support ethtool getstats api.

This returns many of the values that formerly could
only be obtained from debugfs.  This should be an
improvement when trying to access these counters
programatically.  Currently this support is only
enabled when DEBUGFS is enabled because otherwise
these stats are not accumulated.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>