usb-mtp: fix return status of delete

Spotted by Coverity: CID 1399414

mtp delete allows the return status of delete succeeded,
partial_delete or readonly - when none of the objects could be
deleted. Give more meaningful names to return values of the
delete function.

Some initiators recurse over the objects themselves. In that case,
only READ_ONLY can be returned.

Signed-off-by: Bandan Das <bsd@redhat.com>
Message-Id: <20190401211712.19012-2-bsd@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>