mtd: afs: simplify partition parsing

This simplifies the AFS partition parsing to make the code
more straight-forward and readable.

Before this patch the code tried to calculate the memory required
to hold the partition info by adding up the sizes of the strings
of the names and adding that to a single memory allocation,
indexing the name pointers in front of the struct mtd_partition
allocations so all allocated data was in one chunk.

This is overzealous. Instead use kstrdup and bail out,
kfree():ing the memory used for MTD partitions and names alike
on the errorpath.

In the process rename the index variable from idx to i.

Cc: Ryan Harkin <ryan.harkin@linaro.org>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Richard Weinberger <richard@nod.at>