rndis_host: Add link_change function pointer to 'struct rndis_data'.

Callback to signal link state changes from minidriver to
'subminidrivers'.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>