greybus: loopback: fix layer violation

A bundle (protocol) driver has no business creating sysfs entries for an
ancestor device.

Move the sysfs entries to the bundle device for now.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>