brcmfmac: fix interface sanity check

commit 3428fbcd6e6c0850b1a8b2a12082b7b2aabb3da3 upstream.

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: 71bb244b ("brcm80211: fmac: add USB support for bcm43235/6/8 chipsets")
Cc: stable <stable@vger.kernel.org>     # 3.4
Cc: Arend van Spriel <arend@broadcom.com>
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>