usb: gadget: rndis: don't test against req->length

composite.c always sets req->length to zero
and expects function driver's setup handlers
to return the amount of bytes to be used
on req->length. If we test against req->length
w_length will always be greater than req->length
thus making us always stall that particular
SEND_ENCAPSULATED_COMMAND request.

Tested against a Windows XP SP3.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>