ath9k: Remove unnecessary ?: operator

"(thermometer < 0) ? 0 : (thermometer == X)" is equivalent to
"thermometer == X" for X >= 0.

Signed-off-by: Ivan Safonov <insafonov@gmail.com>
[Updated commit message]
Signed-off-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>