ion: secure_buffer: fix iteration of sg_table

We're currently re-using the `i' loop counter to iterate over the chunk
list within our iteration of the secure sg_table. This causes the
iteration of the sg_table to get all messed up and results in crashes
when we walk off the end of the list. Fix it.

Change-Id: I09c84015f8c3cbcb2cea60886241457bb829003c
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>