gpio: mpc8xxx: Fix a resources leak in the error handling path of 'mpc8xxx_probe()'

Commit 698b8eea ("gpio/mpc8xxx: change irq handler from chained to normal")
has introduced a new 'goto err;' at the very end of the function, but has
not updated the error handling path accordingly.

Add the now missing 'irq_domain_remove()' call which balances a previous
'irq_domain_create_linear() call.

Fixes: 698b8eea ("gpio/mpc8xxx: change irq handler from chained to normal")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>