ath9k: fix storage endpoint lookup

commit 0ef332951e856efa89507cdd13ba8f4fb8d4db12 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>