Dave Johnson doesn’t just think about basketball when he’s broadcasting a Washington Wizards game. He also thinks about his mother. His late mother, Mary Lue , suffered from multiple sclerosis and spent most of her time in a wheelchair. For her to leave the house was a chore for all involved; she had to be lifted in and of the car by Dave and his father. On most days, that radio was her only link to the outside world. That radio was usually tuned to WTOP, Washington’s all-news station — a place where her son has worked for the last 20 years. “She listened to WTOP all day long to keep up with the news — and that’s where I ended up working,” the Annapolis resident said. He’s been at the station since 1995. He picked up the Wizards gig two years later — in the 1997-98 basketball season — and is probably best known for that. But he hung on WTOP, too, phoning in early-morning sports reports from wherever his NBA travels take him.