net: ethernet: ti: am65-cpsw-nuss: fix error handling of am65_cpsw_nuss_probe

Convert to using IS_ERR() instead of NULL test for cpsw_ale_create()
error handling. Also fix to return negative error code from this error
handling case instead of 0 in.

Fixes: 93a76530 ("net: ethernet: ti: introduce am65x/j721e gigabit eth subsystem driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>