pinctrl: microchip-sgpio: Correct the fwnode_irq_get() return value check

fwnode_irq_get() may return all possible signed values, such as Linux
error code. Fix the code to handle this properly.

Fixes: be2dc859 ("pinctrl: pinctrl-microchip-sgpio: Add irq support (for sparx5)")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20220906115021.8661-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>