net: dsa: mt7530: switch to use phylink_get_linkmodes()

Switch mt7530 to use phylink_get_linkmodes() to generate the ethtool
linkmodes that can be supported. We are unable to use the generic
helper for this as pause modes are dependent on the interface as
the Autoneg bit depends on the interface mode.

Tested-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>