UBI: fastmap: do not miss bit-flips

The return value from 'ubi_io_read_ec_hdr()' was stored in 'err', not in 'ret'.
This fix makes sure Fastmap-enabled UBI does not miss bit-flip while reading EC
headers, events and scrubs the affected PEBs.

This issue was reported by Coverity Scan.

Artem: improved the commit message.
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

drivers/mtd/ubi/fastmap.c

@@ -423,7 +423,7 @@ static int scan_pool(struct ubi_device *ubi, struct ubi_attach_info *ai,
 				pnum, err);
 			ret = err > 0 ? UBI_BAD_FASTMAP : err;
 			goto out;
-		} else if (ret == UBI_IO_BITFLIPS)
+		} else if (err == UBI_IO_BITFLIPS)
 			scrub = 1;
 
 		/*