ARM: dma-mapping: NULLify dev->archdata.mapping pointer on detach

The current code only clobbers a local variable, so the device is left
with a stale mapping pointer.

Cc: Hiroshi Doyu <hdoyu@nvidia.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Hiroshi Doyu <hdoyu@nvidia.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>