usb: otg: nop: fix oops triggered by otg_register_notifier

This patch adds BLOCKING_INIT_NOTIFIER_HEAD in nop_usb_xceiv_probe,
so that we can avoid oops caused by uninitialized
nop->otg.notifier.rwsem which will be touched in otg_register_notifier
path.

Reported-by: Gupta, Ajay Kumar <ajay.gupta@ti.com>
Tested-by: Gupta, Ajay Kumar <ajay.gupta@ti.com>
Cc: Balbi, Felipe <balbi@ti.com>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>