usb: gadget: move global vars from epautoconf into struct usb_gadget

epautoconf has two global variables which count the endpoint number of
last assigned endpoint.
This patch removes the global variable and keeps it as per (UDC) gadget.
While here, the ifdef is removed and now the in and outpoint are
enumerated unconditionally. The dwc3 for instance supports 32 endpoints
in total.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>