media: i2c: ov9281: Change exposure default value with mode choice

When the exposure range is updated due to changing VBLANK, adopt
the mode default value, not the current default.

Should the specific default value be outside of the min/max range
defined, then the whole update is rejected, which can leave an
invalid exposure range active.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>