net: dsa: vsc73xx: convert to devm_platform_ioremap_resource

Use the helper function that wraps the calls to platform_get_resource()
and devm_ioremap_resource() together.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>