mtd: mtd_oobtest: Fix bitflip_limit usage in test case 3

In test case 3, we set vary_offset to write at different
offsets and lengths in the OOB available area. We need to
do the bitflip_limit check while checking for 0xff outside the
OOB offset + length area that we didn't modify during write.

Signed-off-by: Roger Quadros <rogerq@ti.com>
[Brian: whitespace fixup]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>