mtd: msm_qpic_nand: Use logical unit count in flash density

In an ONFI compliant devices, the flash parameters are calculated
by reading the onfi parameter page of the device.

Existing flash density is calculated based on a single logical
unit(LUN). This shows wrong density information on Multi LUN flashes.
So, always use number of LUNS value in flash density calculations.

Change-Id: Idba4d4e129e7fdcdab1e509bd8c3149f26fc11f2
Signed-off-by: Pradeep P V K <ppvk@codeaurora.org>