[S390] appldata: fix dev_get_stats 64 bit conversion

Fix this warning:

arch/s390/appldata/appldata_net_sum.c: In function 'appldata_get_net_sum_data':
arch/s390/appldata/appldata_net_sum.c:89: warning: initialization from incompatible pointer type

which was introduced with be1f3c2c
"net: Enable 64-bit net device statistics on 32-bit architectures"

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>