gpio/ml-ioh: fix off-by-one for displaying variable i in dev_err

with the "for (; i != 0; i--)" sytax, i-- will be executed after the iteration.
thus dev_err shows wrong i value.
Switch to "while(--i >= 0)" which is better in readability.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>