Entertainer Lola Falana, who quit the stage two years ago because of multiple sclerosis, will play a weekend date at the Sands Hotel in Las Vegas starting Friday. She's going back to work, she says, because "I'm not into being a chicken." Falana, 42, was partially paralyzed when struck by the debilitating disease in 1987. When she was offered the Las Vegas date, she told herself, "It's now or never. . . . You'd be surprised how high you can rise above these things if you set your heart and your mind to it. You have to move ahead, and make things happen."