mailbox: pcc: Reset pcc_chan_count to zero in case of PCC probe failure

Currently, 'pcc_chan_count' is remains set to a non-zero value if PCC
subspaces are parsed successfully but something else fail later during
the initial PCC probing phase. This will result in pcc_mbox_request_channel
trying to access the resources that are not initialised or allocated and
may end up in a system crash.

Reset pcc_chan_count to 0 when the PCC probe fails in order to prevent
the possible issue as described above.

Fixes: ce028702

 ("mailbox: pcc: Move bulk of PCCT parsing into pcc_mbox_probe")
Signed-off-by: Huisong Li <lihuisong@huawei.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>