mtd: msm_qpic_nand: fix nand density variable data type

The density of the NAND device is usually indicated in bits and not
bytes. Currently the variable used to store this information is a
32-bit variable which is not large enough to store the density.
Updating that variable to a 64-bit variable will fix this issue.

Change-Id: Ia1e2c38e46d118c0356f439b73abae14ed95709f
Signed-off-by: Krishna Konda <kkonda@codeaurora.org>