UBI: fix check_data_ff return code

When the data does not contain all 0xFF bytes, 'check_data_ff()' should return
1, not -EINVAL; Also, the caller ('process_eb()') should not add the PEB to the
"corrupted" list if there was a read error.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>