media: tda1997x: Use bitwise or for setting subdev flags

Assigning subdev flags in probe() after v4l2_i2c_subdev_init() clears the
I2C flag set by that function. Fix this by using bitwise or instead.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>