soc: qcom: hab: fix the panic in shmem device shutdown

The global struct list_head pchannels should be directly
used as the list head for pchan list traversal. Previously,
a local struct list_head instance was wrongly created and
used for such traversal. It caused kernel panic in system
shutdown.

Change-Id: Ifb82b55d5a0237fd12f8f53db095fccbcef96fb8
Signed-off-by: Yong Ding <yongding@codeaurora.org>