If devm_platform_ioremap_resource() fails, it never return
NULL pointer, replace the check with IS_ERR().

Fixes: 57bf0f5a ("ARM: pxa: use pdev resource for palmld mmio")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>