gpio: xgs-iproc: fix parsing of ngpios property

of_property_read_u32 returns 0 on success, not true, so we need to
invert the check to actually take over the provided ngpio value.

Fixes: 6a41b6c5

 ("gpio: Add xgs-iproc driver")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>