psm: Swap the unit member in the softc for a device_t.

This entails various changes to make this driver more "modern"
(new-bus vs pre-new-bus) using device_log() and device_printf() rather
than psm%d. It also fixes the device_busy/unbusy calls to use sc->dev
directly rather than looking the device_t up via the devclass and
unit.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D35006

(cherry picked from commit a6c09f20e8e00bec7d9eb088b4f32dd38bf7f490)