ath5k: Fix AR5K_PHY_TXPOWER_RATE_MAX register value setting.

I was reading ath5k power setting code and
noticed typing error in ath5k_hw_txpower function.
Invalid value was written to AR5K_PHY_TXPOWER_RATE_MAX
register.

Signed-off-by: Nikolay Ledovskikh <nledovskikh@gmail.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>