ath9k: Fix RTC reset for AR5416

For AR5416 chipsets, clearing RTC_RESET_EN when setting
the chip to SLEEP mode results in high power consumption.
This patch fixes this issue by not clearing it for AR5416.

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