media: ov5640: fix restore of last mode set

Mode setting depends on last mode set, in particular
because of exposure calculation when downscale mode
change between subsampling and scaling.
At stream on the last mode was wrongly set to current mode,
so no change was detected and exposure calculation
was not made, fix this.

Signed-off-by: Hugues Fruchet <hugues.fruchet@st.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Tested-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>