wcnss: add null check in pm_ops unregister

Add null check for pm_ops in pm_ops unregister to avoid
null pointer dereference.

Conflicts:
	drivers/net/wireless/wcnss/wcnss_wlan.c

Change-Id: I885618a3244b91ba5eddcadb81ae08e965db63b3
CRs-Fixed: 690191
Signed-off-by: Mahesh A Saptasagar <msapta@codeaurora.org>