qeth: Announce tx checksumming for qeth devices in TSO/EDDP mode

TSO requires tx checksumming. For non GSO frames in TSO/EDDP mode we
have to manually calculate the checksum.

Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: Ursula Braun <braunu@de.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>