[MTD][MTDPART] Fix a division by zero bug

When detecting a partition beyond the end of the device, skip most of
the initialisation, in particular those bits causing a division by zero.

Signed-off-by: Jörn Engel <joern@logfs.org>
Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>