Continue searching for an irq map from the start

When searching for a free irq map location continue the search from the
beginning of the list. There may be holes in the map before
irq_map_first_free_idx, e.g. when removing an entries in order will
increase the index past the current free entry.

PR: 271990
Reviewed by: mhorne
Sponsored by: Arm Ltd

Differential Revision: https://reviews.freebsd.org/D40768

(cherry picked from commit 9beb195fd9fdc4beb7d5bca865cf5b19ccd7449b)