s390/qeth: limit csum offload erratum to L3 devices

Combined L3+L4 csum offload is only required for some L3 HW. So for
L2 devices, don't offload the IP header csum calculation.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reference-ID: JUP 394553
Signed-off-by: David S. Miller <davem@davemloft.net>