mtd: nand: use the nand_read_subpage index everywhere

Now that the index variable is correctly set earlier in this function
we can use it in other places that compute the same thing too.

Signed-off-by: Ron Lee <ron@debian.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

diff --cc drivers/mtd/nand/nand_base.c
Simple merge