media: imx274: slightly simplify code

imx274_s_frame_interval() already has a direct pointer to the v4l2
exposure control, so reuse it to simplify code.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>