s390/qeth: enhance TSO control sequence

TSO6 requires the full programming sequence, and not just a simple
START command. This implements the additional ENABLE command, and adds
some sanity checks that were missing for the START command.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>