backlight: adp8860: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/video/backlight/adp8860_bl.c

@@ -690,6 +690,7 @@ static int adp8860_probe(struct i2c_client *client,
 	switch (ADP8860_MANID(reg_val)) {
 	case ADP8863_MANUFID:
 		data->gdwn_dis = !!pdata->gdwn_dis;
+		/* fall through */
 	case ADP8860_MANUFID:
 		data->en_ambl_sens = !!pdata->en_ambl_sens;
 		break;