mtd: docg4: fix oob reads

This patch does two closely related things:

(1) Currently the ecc.read_page() method does not fill the nand->oob_poi buffer
with the oob data, but instead reads oob into a local buffer.  Fix this by
filling the oob_poi buffer instead of a local buffer.  The 'oob_required'
argument is quietly ignored; the device must always read oob after the page
data, and it is presumed that there's no harm in filling oob_poi, even when not
explicitly requested.

(2) Always read oob from the device in ecc.read_oob(), instead of copying it
from a local buffer under some circumstances.

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