nand: docg4: use nand_base's default BBT scan

There's no point in the low level driver doing the work that nand_base
already is doing; just let nand_base set the default BBT scanning
function.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Mike Dunn <mikedunn@newsguy.com>