ath9k: make sure main_rssi is positive

ath9k: make sure main_rssi is positive

some times the rssi control descriptor for the main antenna may be
negative like that of alternate antenna, hence before  incrementing
packet counts/rssi of main/alternate antenna make sure both main_rssi
and alt_rssi are positive only. this avoids wrong selection of antenna
due to diversity

Cc: Gabriel Tseng <[email protected]>
Cc: Senthilkumar Balasubramanian <[email protected]>
Signed-off-by: Mohammed Shafi Shajakhan <[email protected]>
Signed-off-by: John W. Linville <[email protected]>