of/selftest: Fix GPIOs selftest to cover the 7th case

The of_gpio_named_count() self test doesn't hit the out-of-range
condition even though it is coded. Fix the bug by increasing the for
loop range by one.

Reported-by: Andreas Larsson <andreas@gaisler.com>
Cc: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>