[media] r820t: Remove a warning for an unused value

Currently, the driver complains about the pre_detect var:

	drivers/media/tuners/r820t.c: In function 'r820t_sysfreq_sel':
	drivers/media/tuners/r820t.c:722:31: warning: variable 'pre_dect' set but not used [-Wunused-but-set-variable]

While rtl8232 code comments it, perhaps some other driver may use.
So, the better is to keep the code there, allowing to enable it
via r820t config data.
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/tuners/r820t.c

@@ -797,6 +797,13 @@ static int r820t_sysfreq_sel(struct r820t_priv *priv, u32 freq,
 		cable2_in = 0x00;
 	}
 
+
+	if (priv->cfg->use_predetect) {
+		rc = r820t_write_reg_mask(priv, 0x06, pre_dect, 0x40);
+		if (rc < 0)
+			return rc;
+	}
+
 	rc = r820t_write_reg_mask(priv, 0x1d, lna_top, 0xc7);
 	if (rc < 0)
 		return rc;

drivers/media/tuners/r820t.h

@@ -39,6 +39,7 @@ struct r820t_config {
 	enum r820t_chip rafael_chip;
 	unsigned max_i2c_msg_len;
 	bool use_diplexer;
+	bool use_predetect;
 };
 
 #if IS_ENABLED(CONFIG_MEDIA_TUNER_R820T)