Improve HW checksums support in VNIC

• Do not mark CSUM_IP_CHECKED and CSUM_IP_VALID on IPv6 packets. IPv6 does not have checksums by definition.
• Set SCTP packets csum_flags CSUM_SCTP_VALID instead of CSUM_DATA_VALID and skip csum_data
• Set csum_data simply as 0xffff without byteswap

Pointed out by: yongari
Reviewed by: yongari, wma
Obtained from: Semihalf
Sponsored by: Cavium
Differential Revision: https://reviews.freebsd.org/D5537