ath9k_htc: check for underflow in ath9k_htc_rx_msg()

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>