coresight: tmc-etr: fix etr smmu unmap issue

The return value 0 of dma_map_resource() is valid, so should use
the dma_mapping_error() to check for error.

Change-Id: I368f480ed66bad0498e38da2fc6baa78b5935d7f
Signed-off-by: Yuanfang Zhang <zhangyuanfang@codeaurora.org>