mtd: nand: check the return code of 'read_oob/read_oob_raw'

Apparently, there is an implementor of 'read_oob' which may return an
error inidication (e.g. docg4_read_oob may return -EIO).

Test the return value of 'read_oob/read_oob_raw', and if negative,
propagate the error, so it's returned by the '_read_oob' interface.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>