netdev: octeon: fix return value check in octeon_mgmt_init_phy()

In case of error, the function of_phy_connect() returns NULL
pointer not ERR_PTR(). The IS_ERR() test in the return value
check should be replaced with NULL test.

dpatch engine is used to auto generate this patch.
(https://github.com/weiyj/dpatch

)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>