ath9k_htc: Fix RX length check

The length of the received SKB could be equal to
HTC_RX_FRAME_HEADER_SIZE in case of packets with phy/crc errors,
in which case they are dropped without being processed.
Fix this check so that the error counters are updated correctly.

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>