cnss2: Add subsys check when do powerup and powerdown

Currently subsystem register will happen in PCIe enumeration callback.
There are cases that the callback will never be called from PCIe
framework. Add NULL checks for dev from subsys_desc when do powerup
and powerdown to avoid unnecessary crashes.

Change-Id: Ie0df098047f39753d6f904dfcb67be03d18bd56c
Signed-off-by: Yue Ma <yuem@codeaurora.org>