[MTD] OneNAND: Amend write-verify to compare to original buffer

When write-verify is enabled (CONFIG_MTD_ONENAND_VERIFY_WRITE),
the data written is read back and compared.  The comparison
was being made between dataRAM buffers, but this does not
verify that the data made it to the dataRAM correctly in
the first place.  This patch amends write-verify to
compare back to the original buffer.  It also now verifies
sub-page writes.

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