qeth: s390 ethernet device driver dependency

Compile the s390 10GB ethernet device driver only when
ETHERNET has been defined in the kernel configuration file.
Right now the qeth device driver is always built regardless
of which network connectivity is active.

Signed-off-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Signed-off-by: Frank Blaschka <blaschka@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>