cnss: Add NULL check for PM related APIs

Since CMSS PM related APIs can be called at very early stage of WLAN
driver loading, add NULL check for them to avoid crashes if PCIe
enumeration fails which makes the penv variable as NULL.

Change-Id: I7c5e67cb1583b48b24ec04df21e482f4885d2e42
CRs-fixed: 758267
Signed-off-by: Yue Ma <yuem@codeaurora.org>