ath9k_htc: Add ethtool stats support.

This provides some of the same info found in
the ath9k_htc debugfs through the standard ethtool stats API.

This logic is only supported when ath9k_htc debugfs kernel
feature is enabled, since that is the only time stats
are actually gathered.

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