net: ethernet: ti: cpsw: Fix inconsistent IS_ERR and PTR_ERR in cpsw_probe()

Fix inconsistent IS_ERR and PTR_ERR in cpsw_probe,
The proper pointer to use is clk instead of mode.

This issue was detected with the help of Coccinelle.

Fixes: 83a8471b

 ("net: ethernet: ti: cpsw: refactor probe to group common hw initialization")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>