ath9k: Fix bug in delimiter padding computation

There is a roundng error in delimiter padding computation
which causes severe throughput drop with some of AR9003.

signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Cc:stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>