Fix index regression in nand_read_subpage

Commit 7351d3a5 added an index variable
as part of fixing checkpatch warnings, presumably as a tool to make some
long lines shorter, however it only set that index in the case of there
being no gaps in eccpos for the fragment being read.  Which means the
later step of filling ecccode from oob_poi will use the wrong indexing
into eccpos in that case.

This patch restores the behaviour that existed prior to that change.

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