cyclic: drop redundant cyclic_ready flag

We're already relying on gd->cyclic being NULL before cyclic_init() is
called - i.e., we're relying on all of gd being zeroed before entering
any C code. And when we do populate gd->cyclic, its ->cyclic_ready
member is automatically set to true. So we can actually just rely on
testing gd->cyclic itself.

The only wrinkle is that cyclic_uninit() actually did set
->cyclic_ready to false. However, since it doesn't free gd->cyclic,
the cyclic infrastructure is actually still ready (i.e., the list_head
is properly initialized as an empty list).

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-*