usb: usbtest: two super speed fixes for usbtest

bMaxPacketSize0 field for super speed is a power of 2, not a count.
The size itself is always 512.

Max packet size for a super speed bulk endpoint is 1024, so
allocate the urb size in halt_simple() accordingly.

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>