mtd: nand_bbt: use string library

Some nand_bbt code can be shortened by using memcmp() and memchr_inv().
As an added bonus, there is a possible performance benefit.

Borrowed some code from Akinobu Mita.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>