net: phy: phy_support_sym_pause: Clear Asym Pause

When indicating the MAC supports Symmetric Pause, clear the Asymmetric
Pause bit, which could of been already set is the PHY supports it.

Reported-by: Labbe Corentin <clabbe@baylibre.com>
Fixes: c306ad36 ("net: ethernet: Add helper for MACs which support pause")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>