drivers: flash: fix flash-simulator build crash

Statistic for flash has limitation regards number of pages possible to
be counted. This path introduces check for that in source file as it is
not possible to preserve the limit in Kconfig.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
Signed-off-by: Kamil Piszczek <Kamil.Piszczek@nordicsemi.no>