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 <[e-mail]>
Signed-off-by: Babu Ravi <[e-mail]>
Acked-by: David Brownell <[e-mail]>

drivers/usb/otg/nop-usb-xceiv.c

···
 void usb_nop_xceiv_unregister(void)
 {
 	platform_device_unregister(pd);
+	pd = NULL;
 }
 EXPORT_SYMBOL(usb_nop_xceiv_unregister);