mtd: nandsim: make some structures anonymous

We do not need these names. Moreover, there are spelling typos
there: "nansin" instead of "nandsim".

This patch is just a clean up, no functional changes.

Reported-by: Ferenc Wagner <wferi@niif.hu>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>