net: dsa: mt7530: only indicate linkmodes that can be supported

Now that mt7530 is not using the basex helper, it becomes unnecessary to
indicate support for both 1000baseX and 2500baseX when one of the 803.3z
PHY interface modes is being selected. Ensure that the driver indicates
only those linkmodes that can actually be supported by the PHY 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>