usb: gadget: rndis: check size of RNDIS_MSG_SET command

Check the size of the RNDIS_MSG_SET command given to us before
attempting to respond to an invalid message size.

Reported-by: Szymon Heidrich <szymon.heidrich@gmail.com>
Cc: stable@kernel.org
Tested-by: Szymon Heidrich <szymon.heidrich@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
