mtd: nand: move SCANLASTPAGE handling to the correct code block

As nand_default_block_markbad() is becoming more complex, it helps to
have code appear only in its relevant codepath(s). Here, the calculation
of `ofs' based on NAND_BBT_SCANLASTPAGE is only useful on paths where we
write bad block markers to OOB. We move the condition/calculation closer
to the `write' operation and update the comment to more correctly
describe the operation.

The variable `wr_ofs' is also used to help isolate our calculation of
the "write" offset from the usage of `ofs' to represent the eraseblock
offset. This will become useful when we reorder operations in the next
patch.

This patch should make no functional change.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>