ath9k: Move ethtool functions to debug.c

The ethtool statistics are available only when
CONFIG_ATH9K_DEBUGFS is enabled, move these functions
to debug.c

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>