net: dsa: mt7530: move SGMII PCS creation to mt7530_probe function

Move creating the SGMII PCS from mt753x_setup() to the more appropriate
mt7530_probe() function.
This is done also in preparation of moving all functions related to
MDIO-connected MT753x switches to a separate module.

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