net: phylink: fix ksettings_set() ethtool call

While testing a Fiberstore SFP-10G-T module (which uses 10GBASE-R with
rate adaption) in a Clearfog platform (which can't do that) it was
found that the PHYs advertisement was not limited according to the
hosts capabilities when using ethtool to change it.

Fix this by ensuring that we mask the advertisement with the computed
support mask as the very first thing we do.

Fixes: cbc1bb1e

 ("net: phylink: simplify phy case for ksettings_set method")
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>