cxgbe(4): Assume that CSUM_TSO in the transmit path implies CSUM_IP and
CSUM_TCP too.  They are all set explicitly by the kernel usually.

While here, fix an unrelated bug where hardware L4 checksum calculation
was accidentally disabled for some IPv6 packets.

Reported by:	alfred@
MFC after:	3 days

(cherry picked from commit c37d6a03084a22b73b4dd7bfee296ef9cd6054e4)