ath9k: Simplify ar9003_hw_tx99_set_txpower

There's no need to keep the same for loop twice in the code.
Move the txpower cap before the loop to reduce code complexity.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>