dwc: add receive checksum offload for IPv6

This patch adds support for receive checksum offload for TCP/IPv6
and UDP/IPv6. Since receive checksum offload can't be configured
separately for IPv4 and IPv6, IFCAP_RXCSUM and IFCAP_RXCSUM_IPV6
can't be changed independently.

Reviewed by: Timo Völker
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D54756