clk: max77686: Remove unnecessary NULL checking for container_of()

container_of() never returns NULL, thus remove the NULL checking for it.
Also rename get_max77686_clk() to to_max77686_clk() for better readability.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>