clk: Zero the clk_rate_request structure

In order to make sure we don't carry anything over from an already
existing clk_rate_request pointer we would pass to
clk_core_init_rate_req(), let's zero the entire structure before
initializing it.

Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com> # imx8mp
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com> # exynos4210, meson g12b
Signed-off-by: Maxime Ripard <maxime@cerno.tech>