net: dsa: mt7530: introduce mt7530_probe_common helper function

Move commonly used parts from mt7530_probe into new mt7530_probe_common
helper function which will be used by both, mt7530_probe and the
to-be-introduced mt7988_probe.

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