gpio: pxa: checking IS_ERR() instead of NULL

irq_domain_add_legacy() returns NULL on error, it doesn't return error
pointers.

Fixes: 384ca3c6 ('gpio: pxa: change the interrupt management')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>