cnss2: Add NULL check for pci_priv in dev_boot debug handling

If device is not ready, pci_priv will be NULL and be dereferenced in some
test cases using dev_boot debug node. Add NULL check to fix it.

Change-Id: I621d242ec7f34cf7b1eda9e067cf7b222f76eb31
Signed-off-by: Yue Ma <yuem@codeaurora.org>