if_dwc: consistently use if.c helper methods

And if_t rather than struct ifnet *. No functional change intended.

Reviewed by: manu
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35497

(cherry picked from commit ca01879004d4fe49c2617f15048a52281963d0ac)