greybus: es2: fix arpc response-allocation error handling

The wrong pointer was checked for allocation failures when allocating
the ARPC response buffer, something which would lead to allocation
failures going undetected.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>