phy: rockchip: handle clock without enable function

If a clock doesn't supply the enable hook, clk_enable() will return
-ENOSYS.  In this case the clock is always enabled so there is no error
and the phy initialisation should continue.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>