usb: dwc3-msm: Add a NULL check in probe

After retrieving the driver data(dwc3) from platform device(mdwc3),
there is no NULL pointer check for dwc3. If it is NULL may lead to
NULL pointer dereference. Hence add a NULL pointer check to avoid
this.

Change-Id: I2da75863250c086ba975cd99e7cec55db74d4364
Signed-off-by: Azhar Shaikh <azhars@codeaurora.org>