usb ethernet gadget: split RNDIS function

This is a RNDIS function driver, extracted from the all-in-one
Ethernet gadget driver.

Lightly tested ... there seems to be a pre-existing problem when
talking to Windows XP SP2, not quite sure what's up with that yet.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>