s390/qeth: use correct length field in SNMP cmd callback

qeth_snmp_command_cb() is the only cmd callback that pulls the reply's
data length from a low-level transport header field. This requires
additional complexity (ie. reply->offset) to make the header accessible
to what is supposed to be a pure IPA cmd callback.

Adapter cmds have a length field in their sub-cmd header, get the data
length from there instead.

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