greybus: loopback: add missing pr_fmt

Add missing pr_fmt so we can at least tell what module the sole
remaining pr_err was from.

Testing Done:
Tested on DB3.5 with the generic bridge firmware on APB2.

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