Fix iwlan DMA mapping direction

When iwlan runs on IOMMU, IOMMU generates a lot of PTE write faults
because PTE write bit is not set on some of PTE's.  This is because
iwlan driver calls DMA mapping with PCI_DMA_TODEVICE which is read only
in mapping PTE.  But iwlan device actually writes to the mapped page to
update its contents.  This issue is not exposed in swiotlb.  But VT-d
hardware can capture this fault and stop the fault transaction.

The following patch fixes the issue.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Reviewed-by: Bhavesh Davda <bhavesh@vmware.com>
Tested-by: Chris Wright <chrisw@sous-sol.org>
Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>