net: phy: marvell: use phy_modify_changed() for marvell_set_polarity()

Rather than open-coding the phy_modify_changed() sequence, use this
helper in marvell_set_polarity().

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Marek Behún <kabel@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>