mtd: ubi: Use 'max_bad_blocks' to compute bad_peb_limit if available

If the user has not set max_beb_per1024 using either the cmdline or
Kconfig options for doing so, use the MTD function 'max_bad_blocks' to
compute the UBI bad_peb_limit.

Signed-off-by: Jeff Westfahl <jeff.westfahl@ni.com>
Signed-off-by: Zach Brown <zach.brown@ni.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electron.com>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Richard Weinberger <richard@nod.at>