msm: ipa: Fix rndis client disconnection gracefully

if there are packets coming from rndis client to device
IPA driver tries to gives those packets to network driver.
If at the same time USB disconnection is in progress, there
can be a possibility of race where callback is called with NULL
private data and it can result in a undefined behavior at client
driver. Make changes to add proper checks to handle the scenario
gracefully.

Change-Id: I7b3a5731ae21362dc67c53a2de464b17a223d4ba
Signed-off-by: Praveen Kurapati <pkurapat@codeaurora.org>
Signed-off-by: Chaitanya Pratapa <cpratapa@codeaurora.org>