greybus: interface: move boot-status clearing to interface enable

Reading and clearing the boot status of an interface is an interface
operation and belongs in the interface code.

As part of the reworked interface boot sequence, we also want to do this
when enabling (enumerating) a Greybus interface.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>