cyclic: use a flag in gd->flags for recursion protection

As a preparation for future patches, use a flag in gd->flags rather
than a separate member in (the singleton) struct cyclic_drv to keep
track of whether we're already inside cyclic_run().

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Stefan Roese <sr@denx.de>
Tested-by: Tim Harvey <tharvey@gateworks.com> # imx8mm-venice-*