usb: gadget: defer setting maxpacket till ->setup()

maxpacket is set by the udc driver for ep0 very early. This value is
copied by the function gadget used later for the USB_DT_DEVICE and
USB_DT_DEVICE_QUALIFIER query. This seems to work fine so far. For USB3
we need set a different value here. In SS speed it is 2^x with x=9 and
in HS we set something <= 64. If the UDC starts in SS and continues in
HS after the cable has been plugged it will report a too small value.
There setting of this value is defered and taken automaticly from the
ep0 pointer where the UDC driver can update it according to the speed it
detected _after_ a cable has been plugged.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>