mtd: make pagetest work

The mtd_pagetest test did not initialize the pgsize variable, which
basically means it did not work. This problem was reported by
Török Edwin <edwintorok@gmail.com>

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>