mtd: mtd_nandbiterrs: use mtd_test helpers

Use mtdtest_write() and mtdtest_erase_eraseblock() in mtd_test helpers.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Iwo Mergler <Iwo.Mergler@netcommwireless.com.au>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc: Vikram Narayanan <vikram186@gmail.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>