net: dsa: mt7530: generalize read port stats logic

In preparation for migration to use of standard MIB API, generalize the
read port stats logic to a dedicated function.

This will permit to manually provide the offset and size of the MIB
counter to directly access specific counter.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Link: https://patch.msgid.link/20250410163022.3695-2-ansuelsmth@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>