net: dsa: mt7530: split-off common parts from mt7531_setup

MT7988 shares a significant part of the setup function with MT7531.
Split-off those parts into a shared function which is going to be used
also by mt7988_setup.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>