staging: greybus: es2: fix arpc request size

Fix size field of arpc message request by using the header size and not
the pointer size.

Signed-off-by: Rui Miguel Silva <rmfrfs@gmail.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>