[MTD] OneNAND: Select correct chip's bufferRAM for DDP

OneNAND double-density package (DDP) has two chips, each with
their own bufferRAM.  The driver will skip loading data from
the NAND core if the data can be found in a bufferRAM, however
in that case, the correct chip's bufferRAM must be selected
before reading from bufferRAM.

Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>