drivers/iommu: remove unnecessary platform_set_drvdata()

The driver core clears the driver data to NULL after device_release or
on probe failure.  Thus, it is not needed to manually clear the device
driver data to NULL.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Cc: David Brown <davidb@codeaurora.org>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Suman Anna <s-anna@ti.com>
Acked-by: Libo Chen <libo.chen@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>