greybus: loopback: error is an unsigned attribute

The error count is unsigned, so fix the format specifier used in its
attribute definition.

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>