[MTD] mtdpart: Make ecc_stats more realistic.

In the existing implementation, ecc_stats fields are incremented only by
one, regardless of master mtd errors number. For example, if there are N
errors were corrected by ECC, partition ecc_stats.corrected will be
incremented by one.

This commit changes simple increment to sum of old value and parent mtd
error count.

Signed-off-by: Yauhen Kharuzhy <jekhor@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>