MIPS: pci-mt7620: Fix return value check in mt7620_pci_probe()

MIPS: pci-mt7620: Fix return value check in mt7620_pci_probe()

In case of error, the function devm_ioremap_resource() returns
ERR_PTR() and never returns NULL. The NULL test in the return
value check should be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: John Crispin <blogic@openwrt.org>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: linux-mediatek@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12451/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>