fix comparison of ECC stats before and after mtd reads

ECC errors are found by comparing the result of ioctl(ECCGETSTATS)
before and after the read.  But if an error was found causing us to go
to the next block, we'd compare the stats before the *first* read to
the stats after the second (third, fourth, etc.) reads, so we'd read
to the end of the partition without ever succeeding.  Fix logic so we
compare the values before and after each read independently.

Bug: 3162777
Change-Id: I5a13abd7243d2afd1d21bd98cbb233e5124b2e80