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.