mtd: nand: add check for out of page read

When run mtd_oobtest case, there will be one error for step(4),
which turned out it need add one check for out of page read in
nand_do_read_oob just like mtd_do_write_oob did it already.
This commit also fix one typo error for comments in mtd_do_write_oob

Signed-off-by: Jason Liu <r64343@freescale.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>