USB 3.0 defines bMaxPacketSize0 field in the device descriptor as
the exponent of 2, so let's print the calculated actual size.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Stefan Roese <sr@denx.de>