UBI: fix mean EC calculation

(a + b) / (c + d) != a / c + b / d. The old code errornously
assumed this incorrect formuld. Instead, just sum all erase
counters in a 64-bit variable and divide to the number of EBs
at the end.

Thanks to Adrian Hunter for pointing this out.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/drivers/mtd/ubi/scan.h b/drivers/mtd/ubi/scan.h
index 46d444a..966b9b6 100644
--- a/drivers/mtd/ubi/scan.h
+++ b/drivers/mtd/ubi/scan.h
@@ -124,7 +124,7 @@
 	int max_ec;
 	unsigned long long max_sqnum;
 	int mean_ec;
-	int ec_sum;
+	uint64_t ec_sum;
 	int ec_count;
 };