greybus: loopback: use the attribute groups, not group

We should use the attribute groups, not group, for the device, so
add and remove it.  No one should ever be updating a sysfs group for a
device, as that can be pretty dangerous if you don't duplicate _all_
existing attribute for that device, and I don't think we were doing that
here.

Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>