usb: gadget: fastboot: use correct max packet size

Change to use wMaxPacketSize of current speed EP desc for request
length wrap up.

Reviewed-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Li Jun <jun.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>