if_dwc: avoid duplicate packet counts

We already increment the unicast IPACKETS and OPACKETS counters in the
rx/tx paths, respectively. Multicast packets are counted in the generic
ethernet code. Therefore, we shouldn't increment these counters in
dwc_harvest_stats().

Drop the early return from dwc_rxfinish_one() so that we still count
received packets with e.g. a checksum error.

PR: 263817
Reported by: Jiahao LI <jiahali@blackberry.com>
Reviewed by: manu
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35499

(cherry picked from commit 9718759043ec2ef36f12b15963194b866d731b5b)