ath9k: fix ar5416Addac_9100 values

Writing the register at offset 0x98c4 causes a deadlock on the AR913x
SoCs. Although i don't have detailed knowledge about these registers,
but if i change the register offset according to the 'ar5416Addac' table,
it works. Additionally there is no reference to the 0x98c4 elsewhere.

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>