mtd: pxa3xx_nand: initialiaze pxa3xx_flash_ids to 0

pxa3xx_flash_ids wasn't initialized to 0, which in certain cases could
end up containing corrupted values in its members. Fix this to avoid
possible issues.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>