greybus: use outgoing flag when creating operation

In gb_operation_create_common(), a zero response size is still
being used to determine whether to use GFP_KERNEL or GFP_ATOMIC
when allocating a message.  Use the value of the "outgoing"
parameter to decide this instead.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>