net: macb: Fix clk API usage for RISC-V systems

Don't fail in macb_enable_clk() if clk_enable() returns
-ENOSYS because we get -ENOSYS for fixed-rate clocks.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>