Consistently use device_t

Several files use the internal name of struct device instead of
device_t which is part of the public API. This patch changes all
struct device * to device_t.

The remaining occurrences of struct device are those referring to the
Linux or OpenBSD version of the structure, or the code is not built on
FreeBSD and it's unclear what to do.

Submitted by: Matthew Macy <mmacy@nextbsd.org> (previous version)
Approved by: emaste, jhibbits, sbruno
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D7447