clk: test: Switch to clk_hw_get_clk

clk: test: Switch to clk_hw_get_clk

Following the clk_hw->clk pointer is equivalent to calling
clk_hw_get_clk(), but will make the job harder if we need to rework that
part in the future.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220816112530.1837489-2-maxime@cerno.tech
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>