net: phylink: allow ethtool -A to change flow control advertisement

When ethtool -A is used to change the pause modes, the pause
advertisement is not being changed, but the documentation in
uapi/linux/ethtool.h says we should be. Add that capability to
phylink.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>