media: ov5640: fix spurious streamon failures

Time to time, stream on is failing with a strange positive error.
Error code is returned erroneously by ov5640_set_ctrl_exposure()
due to ov5640_get_vts() return value wrongly treated as error.
Fix this by forcing ret to 0 after ov5640_get_vts() success call,
in order that ret is set to success for rest of code sequence.

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