net: fec: handle optional clk_ptp more gracefully

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>