mtd: msm_qpic_nand: update erase page detection

QPIC nand controller's ECC engine does not modify the data that is read
from NAND device during ECC operation. So in case there is an ECC error
on an erased page or codeword read, a register called erased codeword
status is updated.

Fix the erased page detection code to use the above information instead
of trying to look for known patterns that might be present in the data
after ECC operation.

Change-Id: Id478014aa311dfb9fc5564c72fb357b88482c381
Signed-off-by: Krishna Konda <kkonda@codeaurora.org>