ath9k_htc: fix NULL-deref at probe

Make sure to check the number of endpoints to avoid dereferencing a
NULL-pointer or accessing memory beyond the endpoint array should a
malicious device lack the expected endpoints.

Fixes: 36bcce43 ("ath9k_htc: Handle storage devices")
Cc: stable <stable@vger.kernel.org>     # 2.6.39+
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>