video: fbdev: pxafb: Use correct return value for pxafb_probe()

When call function devm_platform_ioremap_resource(), we should use IS_ERR()
to check the return value and return PTR_ERR() if failed.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Cc: Xuefeng Li <lixuefeng@loongson.cn>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1590390705-22898-1-git-send-email-yangtiezhu@loongson.cn