ath9k_hw: Update AR9003 interval to improve 5G Tx EVM

The number of temperature reading samples to average
during a Tx packet is decreased to 1 from 2 to improve
5G Tx EVM with chain 0-only mode.

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>