[media] media: i2c: ths7303: make the pdata as a constant pointer

generally the pdata needs to be a constant pointer in the device
state structure. This patch makes the pdata as a constant pointer
and alongside returns -EINVAL when pdata is NULL.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>