usb: gadget: fix few outdated comments

Fix comments in code to make them up to date.

composite: claiming endpoint is now done by setting ep->claimed flag,
not ep->driver_data.

epautoconf: usb_ep_autoconfig() and usb_ep_autoconfig_ss() return
claimed endpoint with ep->claimed flag already set.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>