mtd: msm_qpic_nand: Fix flash density calculation

Account for the number of logical units (from the
ONFI parameter page) in the flash density calculation
for ONFI complaint devices.

Change-Id: I0dcb35faaaef88a75b46fa12c9443dc9faa0b0fd
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>