net: phy: improve auto-neg emulation in swphy

Auto-neg emulation currently doesn't set bit BMCR_ANENABLE in BMCR,
add this. Users will ignore speed and duplex settings in BMCR because
we're emulating auto-neg, therefore we can remove related code.
See also following discussion [0].

[0] https://marc.info/?t=155041784900002&r=1&w=2



Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>