usb: musb: omap2430: fix compile warning

fix the following compile warning:

drivers/usb/musb/omap2430.c: In function 'musb_otg_notifier_work':
drivers/usb/musb/omap2430.c:279:3: warning: 'return' with a value, in
	function returning void
drivers/usb/musb/omap2430.c:282:2: warning: 'return' with a value, in
	function returning void

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