mtd: nandsim: simplify NS_RAW_OFFSET()

Simplify the definision of NS_RAW_OFFSET() by using (ns)->geom.pgszoob
which holds the sum of page size and OOB size.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>