media: pci: saa7164: replace BUG with error return

It was completely unnecessary to use BUG in saa7164_s_frequency,
just return an error instead. This also solves a smatch error:

drivers/media/pci/saa7164/saa7164-encoder.c:388 saa7164_s_frequency() error: potentially dereferencing uninitialized 'tsport'.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

drivers/media/pci/saa7164/saa7164-encoder.c

@@ -383,7 +383,7 @@ int saa7164_s_frequency(struct saa7164_port *port,
 	else if (port->nr == SAA7164_PORT_ENC2)
 		tsport = &dev->ports[SAA7164_PORT_TS2];
 	else
-		BUG();
+		return -EINVAL; /* should not happen */
 
 	fe = tsport->dvb.frontend;