net: mvpp2: fix phylink handling of invalid PHY modes

[ Upstream commit 0fb628f0f250c74b1023edd0ca4a57c8b35b9b2c ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>