mtd: nandbiterrs: Support for NAND biterrors test on platforms without raw write

Support for NAND biterrors test on platforms without raw write

While the default test mode relies on raw write (mtd_write_oob) to introduce
bit errors into a page, the rewrite test mode doesn't need it.

Changed the overwrite test to use normal writes. The default test mode
is unaffected and still requires raw write as before.

Signed-off-by: Iwo Mergler <Iwo.Mergler@netcommwireless.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>