greybus: es2: fix arpc return value

ARPC should return 0 on success, but instead was returning the number of
jiffies left until the timeout.

This caused cport_reset() to report an error and an incorrect error
message to be printed when disabling a connection.

Reported-by: Alex Elder <elder@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Tested-by: Alex Elder <elder@linaro.org>
Signed-off-by: Alex Elder <elder@linaro.org>