s390/virtio: change virtio_feature_desc:features type to __le32

The feature member of virtio_feature_desc contains little endian
values, given that it contents will be converted with
le32_to_cpu(). The "wrong" __u32 type leads to the sparse warnings
below.
In order to avoid them, use the correct __le32 type instead.

drivers/s390/virtio/virtio_ccw.c:749:14: warning: cast to restricted __le32
drivers/s390/virtio/virtio_ccw.c:762:28: warning: cast to restricted __le32

Acked-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>