net: phy: bcm84881: clear settings on link down

Clear the link partner advertisement, speed, duplex and pause when
the link goes down, as other phylib drivers do.  This avoids the
stale link partner, speed and duplex settings being reported via
ethtool.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>