net: phy: marvell10g: add support for half duplex 100M and 10M

Add support for half-duplex 100M and 10M copper connections by parsing
the advertisment results rather than trying to decode the negotiated
speed from one of the PHYs "vendor" registers.  This allows us to
decode the duplex as well, which means we can support half-duplex mode
for the slower speeds.

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