usb: gadget: composite: fix ep->maxburst initialization

bMaxBurst field on endpoint companion descriptor
is supposed to contain the number of burst minus
1. When passing that to controller drivers, we
should be passing the real number instead (by
incrementing 1).

While doing that, also fix the assumption on
dwc3 that value comes decremented by one.

Signed-off-by: Felipe Balbi <balbi@ti.com>