s390/qeth: reduce data length for ARP cache query

qeth_l3_query_arp_cache_info() indicates a data length that's much
larger than the actual length of its request (ie. the value passed to
qeth_get_setassparms_cmd()). The confusion presumably comes from the
fact that the cmd _response_ can be quite large - but that's no concern
for the initial request IO.

Fixing this up allows us to use the generic qeth_send_ipa_cmd()
infrastructure.

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