staging: greybus: power_supply: fix prop-descriptor request size

Since moving the message buffers off the stack, the dynamically
allocated get-prop-descriptor request buffer is incorrectly sized due to
using the pointer rather than request-struct size when creating the
operation.

Fortunately, the pointer size is always larger than this one-byte
request, but this could still cause trouble on the remote end due to the
unexpected message size.

Fixes: 9d15134d ("greybus: power_supply: rework get descriptors")
Cc: stable <stable@vger.kernel.org>     # 4.9
Cc: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>