mii: Handle link state changes for forced modes in mii_check_media()

mii_check_media() does not update the link (carrier) state or log link
changes when the link mode is forced.  Drivers using the mii library
must do this themselves, but most of them do not.

Instead of changing them all, provide a sensible default behaviour
similar to mii_check_link() when the mode is forced.

via-rhine depends on it being a no-op in this case, so make its call
to mii_check_media() conditional.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>