if_dwc: enable RX checksum offload feature

We claim support in ifcaps, but don't actually enable it.

PR: 263886
Reviewed by: manu
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35498

(cherry picked from commit 35c9edab4169f99de7e8fcada6d9b499c8405f87)