[msm_shared/nand]: Fix bad-block calculation for flash read

Fix calculation of offset according to bad blocks present before the
current page.

Without this change flash read would fail if number of bad blocks before
current_block are more than or equal to difference between current_block
and start_block.

Change-Id: I3c68639a534c0e1ede9c6152b9c0094baede6b55
1 file changed