drivers: can: allow NULL init function

Generic device API allows for NULL init function, do the same for I2C.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

include/zephyr/drivers/can.h

@@ -618,7 +618,11 @@ struct can_device_state {
 		stats_init(&state->stats.s_hdr, STATS_SIZE_32, 7,	\
 			   STATS_NAME_INIT_PARMS(can));			\
 		stats_register(dev->name, &(state->stats.s_hdr));	\
+		if (init_fn != NULL) {					\
 			return init_fn(dev);				\
+		}							\
+									\
+		return 0;						\
 	}
 
 /** @endcond */