net: mvpp2: fix phylink handling of invalid PHY modes

The .validate phylink callback should empty the supported bitmap when
the interface mode is invalid.

Cc: Maxime Chevallier <maxime.chevallier@bootlin.com>
Cc: Antoine Tenart <antoine.tenart@bootlin.com>
Reported-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>