mtd/dataflash.c: fix a problem with the last partition

This patch fix the problem that only the [NB_DATAFLASH_AREA - 1] dataflash
partition can be defined to use the area to the end of dataflash size.
Now it is possible to have only one dataflash partition from 0 to the end
of of dataflash size.

Signed-off-by: Ilko Iliev <iliev@ronetix.at>