mdio: Advertise pause (flow control) settings even if autoneg is off

Currently, if pause autoneg is off we do not set either pause
advertising flag.  If autonegotiation of speed and duplex settings is
enabled, there is no way for the link partner to distinguish this from
our refusing to use pause frames.

We should instead set the advertising flags according to the forced
mode so that the link partner can follow our lead.  This is consistent
with the behaviour of other drivers.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>