firewire: Initialize firewire_devclass in fw_modevent.

[jhb]

Summary

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.

[emaste]

Is it time to look at deprecation plans for firewire?

[jhb]

I thought it was already deprecated which is why I didn't try to fix this "cleanly".