icnss: Ignore PD down if WLAN FW doesn't exists

During Modem SSR, PD down notification gets notified by
service-notifier. There are cases where in WLAN firmware may not
be available on Modem side. Handle PD down notification
gracefully in case WLAN firmware doesn't exists.

CRs-fixed: 1072772
Change-Id: Ibfe29a3950fabfcf8baa5d08b15e0f056dcc1f2a
Signed-off-by: Prashanth Bhatta <bhattap@codeaurora.org>