BugLink: https://bugs.launchpad.net/bugs/1878232

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ian May <ian.may@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>