usb: musb: omap2430: minor cleanups.

1/ remove incorrect comment (it is a non-blocking notifier)
2/ Use correct symbolic return value for notifier
3/ Make sure otg_notifier_work is cancelled before module exit.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>