net: ethernet: ti: fix some return value check of cpsw_ale_create()

cpsw_ale_create() can return both NULL and PTR_ERR(), but all of
the caller only check NULL for error handling. This patch convert
it to only return PTR_ERR() in all error cases, and the caller using
IS_ERR() instead of NULL test.

Fixes: 4b41d343 ("net: ethernet: ti: cpsw: allow untagged traffic on host port")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>