[media] st-delta: mjpeg: fix static checker warning

Initialize 'data_offset' local variable to 0.

drivers/media/platform/sti/delta/delta-mjpeg-dec.c:415 delta_mjpeg_decode()
error: uninitialized symbol 'data_offset'.

Fixes: 433ff5b4: "[media] st-delta: add mjpeg support"

Signed-off-by: Hugues Fruchet <hugues.fruchet@st.com>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>