USB: otg: fix module reinsert issue

Platform_device instance (pd) is not set to NULL in
usb_nop_xceiv_unregister() causing usb_nop_xceiv_register()
to fail during module reinsert.

From: Ajay Kumar Gupta <ajay.gupta@ti.com>
Signed-off-by: Babu Ravi <ravibabu@ti.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>