mailbox: qcom-ipcc: Fix IPCC mbox channel exhaustion

Fix IPCC (Inter-Processor Communication Controller) channel exhaustion by
setting the channel private data to NULL on mbox shutdown.

Err Logs:
remoteproc: MBA booted without debug policy, loading mpss
remoteproc: glink-edge: failed to acquire IPC channel
remoteproc: failed to probe subdevices for remoteproc: -16

Fixes: fa74a025 ("mailbox: Add support for Qualcomm IPCC")
Signed-off-by: Sibi Sankar <sibis@codeaurora.org>
Cc: stable@vger.kernel.org
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>