media: imx290: mark read reg function as __always_unused

Such function is currently unused, but could be needed in the
future. So, keep it, marking it as __always_unused, in order to
avoid a clang W=1 error.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/i2c/imx290.c

@@ -363,7 +363,7 @@ static inline struct imx290 *to_imx290(struct v4l2_subdev *_sd)
 	return container_of(_sd, struct imx290, sd);
 }
 
-static inline int imx290_read_reg(struct imx290 *imx290, u16 addr, u8 *value)
+static inline int __always_unused imx290_read_reg(struct imx290 *imx290, u16 addr, u8 *value)
 {
 	unsigned int regval;
 	int ret;