mtd: mtd_subpagetest: use mtd_test helpers

mtd: mtd_subpagetest: use mtd_test helpers

Use mtdtest_scan_for_bad_eraseblocks() and mtdtest_erase_good_eraseblocks()
in mtd_test helpers.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc: Vikram Narayanan <vikram186@gmail.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Artem Bityutskiy <dedekind1@gmail.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>