soc: qcom: spcom: lock ion buf, decrement ref count if no free entry

spcom_handle_lock_ion_buf_command() should decrement ref count,
that is incremented by ion_import_dma_buf().

Change-Id: Ic7f814bd2791b4dbcf6356f64ad32ee85c6cb103
Signed-off-by: Amir Samuelov <amirs@codeaurora.org>