ath9k: remove (u16) casts from rtc register access

The RTC register offsets don't fit into 'u16' on the AR913x, so we have
to remove the existing casts.

Changes-licensed-under: ISC

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Tested-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>