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>