usb: musb: mediatek: don't unregister something that wasn't registered

This function only calls mtk_otg_switch_init() when the ->port_mode
is MUSB_OTG so the clean up code should only call mtk_otg_switch_exit()
for that mode.

Fixes: 0990366b ("usb: musb: Add support for MediaTek musb controller")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Link: https://lore.kernel.org/r/Y8/3TqpqiSr0RxFH@kili


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>