ixgbe: cleanup ethtool autoneg input

The way we were setting autoneg via ethtool was inconstant with that
of our other drivers.  It will change the following:

If autoneg is off:
>ethtool -a eth0
Pause parameters for eth0:

Autonegotiate:  off
RX:             off
TX:             off

Before:
>ethtool -A eth0 autoneg on
>ethtool -a eth0
Pause parameters for eth0:

Autonegotiate:  off
RX:             off
TX:             off

Now:
>ethtool -A eth0 autoneg on
>ethtool -a eth0
Pause parameters for eth0:

Autonegotiate:  on
RX:             on
TX:             on

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>