firewire: Initialize firewire_devclass in fw_modevent.

The use of devclass_get_softc() combined with cdev unit numbers is
probably not ideal (probably should be initializing si_drv1 when each
cdev is created instead), but it looks like a bit of a PITA to do, so
just initialize the devclass explicitly instead.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D35060