rndis_host: Split up rndis_host.c

Split up rndis_host.c into rndis_host.h and rndis_base.c. This is done so
that rndis_wlan can reuse common parts with rndis_host.

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>