net: fec: drop check for clk==NULL before calling clk_*

clk_prepare, clk_enable and their counterparts (at least the common clk
ones, but also most others) do check for the clk being NULL anyhow (and
return 0 then), so there is no gain when the caller checks, too.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>