gpio: Fix range check in of_gpio_simple_xlate()

of_gpio_simple_xlate() has an off-by-one bug where it checks to see if
args[0] is > ngpio instead of >=.  args[0] must always be less than
ngpio because it is a zero-based enumeration.

Signed-off-by: Roland Stigge <stigge@antcom.de>
[grant.likely: beef up commit text]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>