s390/qeth: make OSN / OSX support configurable

The last machine generation that supports OSN is z13, and OSX is only
supported up to z14. Allow users and distros to decide whether they
still need support for these device types.

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