drivers: can: remove CAN_BUS_UNKNOWN CAN controller state

The CAN_BUS_UNKNOWN CAN controller state is only used to indicate that
the current CAN controller state could not be read.

Remove it and change the signature of the can_get_state() API function
to return an integer indicating success or failure.

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