net: mvneta: use devm_platform_ioremap_resource() to simplify code

devm_platform_ioremap_resource() wraps platform_get_resource() and
devm_ioremap_resource() in a single helper, let's use that helper to
simplify the code.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Signed-off-by: David S. Miller <davem@davemloft.net>