rtl8xxxu: fix interface sanity check

Make sure to use the current alternate setting when verifying the
interface descriptors to avoid binding to an invalid interface.

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: 26f1fad2 ("New driver: rtl8xxxu (mac80211)")
Cc: stable <stable@vger.kernel.org>     # 4.4
Cc: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>