UBI: check max_beb_per1024 value in ubi_attach_mtd_dev

max_beb_per1024 shouldn't be negative, and a 0 value will be treated as
the default value. For the upper bound, 768/1024 should be enough.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>