mtd: tests: fix read, speed and stress tests on NOR flash

Before using block_isbad() check if mtd->block_isbad() is defined.
Calculating pgcnt must be done using pgsize defined to 512 on
NOR and mtd->writesize for NAND, not using  mtd->writesize directly.

Signed-off-by: Morten Thunberg Svendsen <mts.doredevelopment@gmail.com>
Acked-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>