spibus(4): Allow IRQ resource to be released

IRQ Resource is allocated on spibus(4). We must release it here too
rather than propagate request down the tree.

Fixes: 4dd8db62e9d3 ("Add IRQ resource to SPIBUS")

MFC after: 1 week
Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D41243