greybus: es2: always set result value

Make sure to always set the result value for ARPC instead of forcing
every caller to do it in order to avoid compiler warnings.

The ARPC result should still be ignored unless arpc_sync returns
-EREMOTEIO.

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