ath9k: fix storage endpoint lookup

Make sure to use the current alternate setting when verifying the
storage interface descriptors to avoid submitting an URB to an invalid
endpoint.

Failing to do so could cause the driver to misbehave or trigger a WARN()
in usb_submit_urb() that kernels with panic_on_warn set would choke on.

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@codeaurora.org>