net: usb: ax88179_178a: Use linkmode helpers for EEE

Make use of the existing linkmode helpers for converting PHY EEE
register values into links modes, now that ethtool_keee uses link
modes, rather than u32 values.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>