cyclic: make cyclic_unregister() idempotent

cyclic: make cyclic_unregister() idempotent

Make cyclic_unregister() safe to call with an already unregistered, or
possibly never registered, struct cyclic_info. This is similar to how
the various timer APIs in the linux kernel work (they all allow
calling delete/cancel/... on an inactive timer object).

This means callers don't have to separately keep track of whether
their cyclic callback is registered or not, and avoids them trying to
peek into the struct cyclic_info for that information - which leads to
somewhat ugly code as it would have to be guarded by ifdef
CONFIG_CYCLIC.

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
Reviewed-by: Stefan Roese <sr@denx.de>