soc: qcom: spcom: fix unlock ion buffer

The buffer is referenced by 'fd' supplied by user space. Lock/Unlock
functionality adds/removes reference on underlying dma buffer. 'fd' is
mapped to dma_buf pointer by dma buf framework. After lock every dma
buffer tracked in spcom driver (per channel).

This change fixes unlock operation by look up underlying dma_buf pointer
and doing unlock for the dma buffer pointer stored before (by lock
operation), instead comparing only 'fd' value (which may be re-mapped
after lock by userspace).

Change-Id: I11cf9399eef2607c31ab1a9deb655fdd90d99d22
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>