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commit 3a318426 upstream.

We check for overflow here, but we don't check for underflow so it
causes a static checker warning.

Fixes: fb9987d0

 ('ath9k_htc: Support for AR9271 chipset.')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>