msm: mdm: Fix bug in retrieving driver's private data

The current code retrieves driver's private data based on platform
device id or device file name.  The device id match is broken
for targets which has only one external modem (platform device
id = -1).

Use device's private data and file's private data fields for
storing the driver's private data.

CRs-Fixed: 451830
Change-Id: I080ef86cce8d346c0b3e380432712dde07b64aea
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>