net: phy: marvell: Use the set_polarity helper

Some of the init functions unilaterally enable set auto cross over
without using the helper. Make use of the helper, and respect the
phydev MDI configuration.

Clean up the #define used while setting polarity, and the other
functions of the bits in the register.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>