net: fddi: fix a possible null-ptr-deref

bp->SharedMemAddr is set to NULL while bp->SharedMemSize lesser-or-equal 0,
then memset will trigger null-ptr-deref.

fix it by replacing pci_alloc_consistent with dma_zalloc_coherent.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>