mtd: mtd_nandecctest: make module_init() return an error code if test fails

Return an error code if test fails in order to detect a test case failure
by invoking tests repeatedly like this:

while sudo modprobe mtd_nandecctest; do
	sudo modprobe -r mtd_nandecctest
done

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>