soc: qcom: secure_buffer: Add missing dma_unmap_single

Add a missing dma_unmap_single() that was accidentally deleted when adding
tracepoints in the secure_buffer driver.

Change-Id: I8159bee354b4debffb2a55165d2e543d5979daff
Signed-off-by: Chris Goldsworthy <cgoldswo@codeaurora.org>