mtd: parsers: afs: Fix freeing the part name memory in failure

In the case of failure while parsing the partitions, the iterator should
be pre decremented by one before starting to free the memory allocated
by kstrdup(). Because in the failure case, kstrdup() will not succeed
and thus no memory will be allocated for the current iteration.

Cc: Linus Walleij <linus.walleij@linaro.org>
Fixes: 1fca1f6a ("mtd: afs: simplify partition parsing")
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>