bnxt: add dma mapping attributes

On the SPARC platform we need to use the DMA_ATTR_WEAK_ORDERING attribute
in our Rx path dma mapping in order to get the expected performance out
of the receive path.  Adding it to the Tx path has little effect, so
that's not a part of this patch.

Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Reviewed-by: Tushar Dave <tushar.n.dave@oracle.com>
Reviewed-by: Tom Saeger <tom.saeger@oracle.com>
Acked-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>