greybus: loopback: Fix broken loopback min values

Currently, when a loopback test completely fail,
loopback will return 4294967295 for every min value.
Return 0 instead of 4294967295 in such case.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>