clk: Switch from __clk_determine_rate to clk_core_round_rate_nolock

clk_mux_determine_rate_flags() will call into __clk_determine_rate()
with a clk_hw pointer, while it has access to the clk_core pointer
already.

This leads to back and forth between clk_hw and clk_core, while
__clk_determine_rate will only call clk_core_round_rate_nolock() with
the clk_core pointer it retrieved from the clk_hw.

Let's simplify things a bit by calling into clk_core_round_rate_nolock
directly.

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>