s390/qeth: do basic setup for data channel

The data channel currently doesn't need a setup operation, because we
don't use pre-allocated cmd buffers for its IO. But subsequent changes
will introduce further setup that also applies to the data channel.
This refactors things a bit, so that the new stuff can then be
automatically applied to all channels.

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