net: mdio: aspeed: move reg accessing part into separate functions

Add aspeed_mdio_op() and aseed_mdio_get_data() for register accessing.

aspeed_mdio_op() handles operations, write command to control register,
then check and wait operations is finished (bit 31 is cleared).

aseed_mdio_get_data() fetchs the result value of operation from data
register.

Signed-off-by: Potin Lai <potin.lai@quantatw.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>