greybus: loopback: all read-only attributes are unsigned

The only values passed to the gb_loopback_ro_attr() macro are
unsigned 32-bit values.  So there's no need to pass a "type"
format specifier.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>