mtd: mtdoops: refactor loop

We can clean up the loop logic a bit, here. This refactoring was enabled
in part by:

 Commit bb4a0986 [mtdoops: clean-up new MTD API usage]

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>