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 <Gabriel.Tseng@Atheros.com>
Cc: Senthilkumar Balasubramanian <Senthilkumar.Balasubramanian@Atheros.com>
Signed-off-by: Mohammed Shafi Shajakhan <mshajakhan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>