rocker: do not check gen bit when getting desc for out rings

It makes no sense to check it. Hw will negate the previous value and we
only need to ensure it is set to 0 before we write HEAD.

Reported-by: Scott Feldman <sfeldma@cumulusnetworks.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Scott Feldman <sfeldma@cumulusnetworks.com>