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

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.

Fixes: e88500247dc3 ("gpio: add QIXIS FPGA GPIO controller")
Signed-off-by: Felix Gu <ustc.gu@gmail.com>
Link: https://patch.msgid.link/20260320-qixis-v1-1-a8efc22e8945@gmail.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>