ath9k: remove variable which is set but never read

No functional changes - this only removes a variable which is set but
never read.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>