init.usb.rc: set sys.usb.config to none before changing it

On encrypted devices, persistent properties are loaded after the device
is decrypted. To properly change sys.usb.config to its persistent value,
it must first be set to "none" and then to ${persist.sys.usb.config}.

Bug: 7678835
Change-Id: I4f91245cedc04e3178ce9cee21390f5b657733c9