of/irq: Avoid calling list_first_entry() for empty list

list_first_entry() expects the list is not empty, we need to check if list is
empty before calling list_first_entry(). Thus use list_first_entry_or_null()
instead of list_first_entry().

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>