drivers: net: canbus: move CAN bus network driver to drivers/net

Move the CAN bus network driver from drivers/can to drivers/net as it
implements a network driver, not a CAN controller driver.

Use a separate Kconfig for enabling the CAN bus network driver instead of
piggybacking on the SocketCAN Kconfig. This allows for other
(e.g. out-of-tree) SocketCAN transports.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>