MFC r322596:
Add SI_SUB_TASKQ after SI_SUB_INTR and move taskqueue initialization there for EARLY_AP_STARTUP

This fixes a regression accidentally introduced in r322588, due to an
interaction with EARLY_AP_STARTUP.

Discussed with: cem@
Reviewed by: bdrewery@, jhb@
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D12053