bcm_dma: don't dereference NULL softc

This file defines a small API to be used by other drivers. If any of
these functions are called before the bcm_dma device has attached we
should handle the error gracefully. Fix a formatting quirk while here.

Reviewed by: manu
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D38756

(cherry picked from commit 07ccf71451d7377b1a6f3367f738ce7ddb1f2a24)