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).

Changes since v2 [1]:
* Merged with latest code changes

Changes since v1:
Update made thanks to David's review, much appreciated David.
* Improved inconsistent failure handling of clock rate setting
* For completeness of usecase, added arc_emac_probe error handling

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