net/fec: set con_id in clk_get() call to NULL

The con_id is actually not needed for clk_get().

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>