mtd: nand: pxa3xx: Clear cmd buffer #3 (NDCB3) on command start

Command buffer #3 is not properly cleared and it keeps the last
set value. Fix this by clearing when a command is setup.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>