msm: ipa: add a NULL check for IPA WAN netdevice in SSR

The change adds a NULL check to IPA WAN driver before invoking
netif_stop_queue in Modem SSR procedure. This prevents issues
when the Modem pre-shutdown indication is received multiple times
consecutively.

Change-Id: I2385923cf7ff4bb932e7799188410428d4ed08e6
Signed-off-by: Sivan Reinstein <sivanr@codeaurora.org>
Acked-by: David Arinzon <darinzon@qti.qualcomm.com>