net: ethernet: arc: fix error handling in emac_rockchip_probe

If clk_set_rate() fails, we should disable clk before return.
Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Branislav Radocaj <branislav@radocaj.org>
Signed-off-by: David S. Miller <davem@davemloft.net>