rndis_host: export functions

Export rndis_host functions and also rename rndis_bind() to
generic_rndis_bind() for modules using rndis_host as base.

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>