ARM: OMAP: replace checks for CONFIG_USB_GADGET_OMAP

Commit 193ab2a6 ("usb: gadget: allow multiple gadgets to be built")
apparently required that checks for CONFIG_USB_GADGET_OMAP would be
replaced with checks for CONFIG_USB_OMAP. Do so now for the remaining
checks for CONFIG_USB_GADGET_OMAP, even though these checks have
basically been broken since v3.1.

And, since we're touching this code, use the IS_ENABLED() macro, so
things will now (hopefully) also work if USB_OMAP is modular.

Fixes: 193ab2a6 ("usb: gadget: allow multiple gadgets to be built")
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Tony Lindgren <tony@atomide.com>