s390/qeth: phase out OSN support

OSN devices currently spend an awful long time in qeth_l2_set_online()
until various unsupported HW cmds time out. This has been broken for
over two years, ever since
commit d22ffb5a ("s390/qeth: fix IPA command submission race")
triggered a FW bug in cmd processing.
Prior to commit 782e4a79 ("s390/qeth: don't poll for cmd IO completion"),
this wait for timeout would have even been spent busy-polling.

The offending patch was picked up by stable and all relevant distros,
and yet noone noticed.
OSN setups only ever worked in combination with an out-of-tree blob, and
the last machine that even offered HW with OSN support was released back
in 2015.

Rather than attempting to work-around this FW issue for no actual gain,
add a deprecation warning so anyone who still wants to maintain this
part of the code can speak up. Else rip it all out in 2021.

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