greybus: operation: fix operation ordering

Make the operation work queue single threaded.

The operation work queue was meant to be single threaded, but due to a
missing flag instead allowed one active task per CPU, something which
could lead to requests being processed out of order on SMP systems.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Tested-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>