V4L/DVB (4291): Add dvbpll i2c device check.

Some cards have multiple possible addresses for their PLLs, with no other
way to tell if a PLL is present or not apart from probing to see if an i2c
device is present. This adds a quick check to see if an i2c device is
present at the given i2c address.
Signed-off-by: Andrew de Quincey <adq_dvb@lidskialf.net>
Acked-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>

drivers/media/dvb/frontends/dvb-pll.c

@@ -613,7 +613,20 @@ static struct dvb_tuner_ops dvb_pll_tuner_ops = {
 
 int dvb_pll_attach(struct dvb_frontend *fe, int pll_addr, struct i2c_adapter *i2c, struct dvb_pll_desc *desc)
 {
+	u8 b1 [] = { 0 };
+	struct i2c_msg msg [] = { { .addr = pll_addr, .flags = 0, .buf = NULL, .len = 0 },
+				  { .addr = pll_addr, .flags = I2C_M_RD, .buf = b1, .len = 1 } };
 	struct dvb_pll_priv *priv = NULL;
+	int ret;
+
+	if (fe->ops.i2c_gate_ctrl)
+		fe->ops.i2c_gate_ctrl(fe, 1);
+
+	ret = i2c_transfer (i2c, msg, 2);
+	if (ret != 2)
+		return -1;
+	if (fe->ops.i2c_gate_ctrl)
+		     fe->ops.i2c_gate_ctrl(fe, 0);
 
 	priv = kzalloc(sizeof(struct dvb_pll_priv), GFP_KERNEL);
 	if (priv == NULL)