greybus: operation: fix atomic message submission

The recently added GFP-flags argument to gb_message_send was never used.

Fixes: 9218fac2a24d ("operation: allow atomic request submissions")
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>