libmtd: allow write operations when MEMWRITE is not supported

libmtd: allow write operations when MEMWRITE is not supported

MEMWRITE is a recently introduced write interface for MTD; however, it
is only supported on NAND flash. mtd-utils should fall back to
old write methods when either ENOTTY or EOPNOTSUPP are returned.

This is a showstopper when, for instance, using ubiformat on NOR, which
don't have a mtd->write_oob interface (and thus don't support MEMWRITE):

  ubiformat: formatting eraseblock 2 --  1 % complete  libmtd: error!: MEMWRITE ioctl failed for eraseblock 2 (mtd3)
          error 122 (Operation not supported)

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>