ibmvnic: substitute mb() with dma_wmb() for send_*crq* functions

The CRQ and subCRQ descriptors are DMA mapped, so dma_wmb(),
though weaker, is good enough to protect the data structures.

Signed-off-by: Lijun Pan <ljp@linux.ibm.com>
Acked-by: Thomas Falcon <tlfalcon@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>