stmmac: fix ptr_ret.cocci warning

drivers/net/ethernet/stmicro/stmmac/stmmac_platform.c:304:1-3: WARNING: PTR_ERR_OR_ZERO can be used

Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Generated by: scripts/coccinelle/api/ptr_ret.cocci

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
[je: rebase and insert newline before return]
Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>