[CAN]: Clean up module auto loading

Remove local char array to construct module name.
Don't call request_module() when CONFIG_KMOD is not set.

Signed-off-by: Urs Thuermann <urs.thuermann@volkswagen.de>
Signed-off-by: Oliver Hartkopp <oliver.hartkopp@volkswagen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>