vmxnet3: set the DMA mask before the first DMA map operation

The DMA mask must be set before, not after, the first DMA map operation, or
the first DMA map operation could in theory fail on some systems.

Fixes: b0eb57cb ("VMXNET3: Add support for virtual IOMMU")
Signed-off-by: Regis Duchesne <hpreg@vmware.com>
Acked-by: Ronak Doshi <doshir@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>