mtd: diskonchip: don't call nand_scan_bbt() directly

The diskonchip driver almost uses the default nand_base hooks as-is,
except that it provides custom on-flash BBT descriptors and avoids using
factory-marked bad blockers.

So let's refactor the BBT initialization code into a private 'late_init'
hook which handles all the private details. Note the usage of
NAND_SKIP_BBTSCAN, which allows us to defer the BBT scan until we've
prepared everything.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>