gpio: qixis-fpga: Fix error handling for devm_regmap_init_mmio()

In the Linux kernel, the following vulnerability has been resolved: gpio: qixis-fpga: Fix error handling for devm_regmap_init_mmio() devm_regmap_init_mmio() returns an ERR_PTR() on failure, not NULL. The original code checked for NULL which would never trigger on error, potentially leading to an invalid pointer dereference. Use IS_ERR() and PTR_ERR() to properly handle the error case.