usb: gadget: composite: fix bMaxPacketSize for SuperSpeed

For bMaxPacketSize0 we usually take what is specified in ep0->maxpacket.
This is fine in most cases, however on SuperSpeed bMaxPacketSize0
specifies the exponent instead of the actual size in bytes. The only
valid value on SS is 9 which denotes 512 bytes.

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