[media] mt9v032: fix uninitialized variable warning

drivers/media/i2c/mt9v032.c: In function ‘mt9v032_probe’:
  CC [M]  drivers/media/i2c/s5k4ecgx.o
drivers/media/i2c/mt9v032.c:996:20: warning: ‘pdata’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  if (pdata && pdata->link_freqs) {
                    ^

It can indeed be uninitialized in one corner case. Initialize to NULL.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>