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 3c760430882730431f5870f23294cb580e98ea3f)
(cherry picked from commit 8b93da4badabc5832ea6d5d72e6a477277e3d43b)