mtd: do use mtd->point directly

Remove direct usage of the "mtd->point" function pointer. Instead,
test the mtd_point() return code for '-EOPNOTSUPP'.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>