mtd: mtdoops: skip reading initially bad blocks

Use block_isbad to check and skip the bad blocks reading.
This will allow to get rid of the read errors if bad blocks
are present initially.

Cc: stable@kernel.org
Signed-off-by: Roman Tereshonkov <roman.tereshonkov@nokia.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>