ARM: 8835/1: dma-mapping: Clear DMA ops on teardown

Installing the appropriate non-IOMMU DMA ops in arm_iommu_detch_device()
serves the case where IOMMU-aware drivers choose to control their own
mapping but still make DMA API calls, however it also affects the case
when the arch code itself tears down the mapping upon driver unbinding,
where the ops now get left in place and can inhibit arch_setup_dma_ops()
on subsequent re-probe attempts.

Fix the latter case by making sure that arch_teardown_dma_ops() cleans
up whenever the ops were automatically installed by its counterpart.

Reported-by: Tobias Jakobi <[e-mail]>
Reported-by: Marek Szyprowski <[e-mail]>
Fixes: 1874619a7df4 "ARM: dma-mapping: Set proper DMA ops in arm_iommu_detach_device()"
Tested-by: Tobias Jakobi <[e-mail]>
Tested-by: Thierry Reding <[e-mail]>
Signed-off-by: Robin Murphy <[e-mail]>
Signed-off-by: Russell King <[e-mail]>

arch/arm/mm/dma-mapping.c

···
 		return;
 
 	arm_teardown_iommu_dma_ops(dev);
+	/* Let arch_setup_dma_ops() start again from scratch upon re-probe */
+	set_dma_ops(dev, NULL);
 }