greybus: es2: allow ARPC result to be ignored

Allow user of ARPC to pass a NULL pointer for the ARPC result.

For simple requests there may only be one error code for remote errors,
or the caller may simply not care to differentiate them.

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