Revert "cxgbe(4): Assume that CSUM_TSO in the transmit path implies CSUM_IP and"

This reverts commit d906f3fd22f65677dabef61c954fd3699ef57586.
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)