Adjust the vertical blanking when s_frame_interval is used.

s_frame_interval allows to set a fixed frame rate, which gets stored as
the sensor's current one. When a new mode is applied, the current frame
rate is reset to the mode's default one. In order to correctly report
the currently configured vertical blanking for s_frame_interval users,
verify that the desired frame rate has not been changed. If that's the
case, compute the correct blanking value and reflect it through the
corresponding v4l2 control.
Signed-off-by: Hugues Fruchet <hugues.fruchet@st.com>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>