dmaengine: qcom: fix gpi undefined behavior

gcc points out an incorrect error handling loop:

drivers/dma/qcom/gpi.c: In function 'gpi_ch_init':
drivers/dma/qcom/gpi.c:1254:15: error: iteration 2 invokes undefined behavior [-Werror=aggressive-loop-optimizations]
 1254 |  struct gpii *gpii = gchan->gpii;
      |               ^~~~
drivers/dma/qcom/gpi.c:1951:2: note: within this loop
 1951 |  for (i = i - 1; i >= 0; i++) {
      |  ^~~

Change the loop to correctly walk backwards through the
initialized fields rather than off into the woods.

Fixes: 5d0c3533

 ("dmaengine: qcom: Add GPI dma driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210103135738.3741123-1-arnd@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>