ARM: OMAP1: fix omap_udc registration

omap_udc platform device is not registered properly anymore:
CONFIG_USB_GADGET_OMAP was deleted by 193ab2a6

 (usb: gadget: allow
multiple gadgets to be built) already in v3.1.

Fix by using CONFIG_USB_OMAP instead. Tested on Nokia 770 by checking
that omap_udc is probed & working properly when built as a module.

Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Tony Lindgren <tony@atomide.com>