Move usbnet.h and rndis_host.h to include/linux/usb

Move headers usbnet.h and rndis_host.h to include/linux/usb and fix includes
for drivers/net/usb modules. Headers are moved because rndis_wlan will be
outside drivers/net/usb in drivers/net/wireless and yet need these headers.

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>