[MTD] mtdoops: fix a bit of spin lock usage

- do not leave spin lock locked
- initialise spin lock

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Acked-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>