net: ethoc: Remove useless test before clk_disable_unprepare

clk_disable_unprepare() already checks that the clock pointer is valid.
No need to test it before calling it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>