net: dsa: mt7530: introduce mt7530_remove_common helper function

Move commonly used parts from mt7530_remove into new
mt7530_remove_common helper function which will be used by both,
mt7530_remove and the to-be-introduced mt7988_remove.

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