arcmsr: Store softc pointer in si_drv1 of cdev.

Rather than fetching the softc using the controller's unit number as
an index into the devclass.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D35004

(cherry picked from commit a9e5e04e6468e7ad9d17a8fe73b2bc7091894934)