When the optional clk_ptp is absent, we can just set it to NULL, and
clk API will just handle it gracefully. It saves us from checking
clk_ptp whenever calling into clk API.
Also since clk_ptp is optional, the "ret" variable shouldn't be set
in case that the clock is absent.
Signed-off-by: Shawn Guo <shawn.guo@linaro.org> Acked-by: David S. Miller <davem@davemloft.net>