ASoC: mxs-sgtl5000: Configure the dai_links as unidirectional

On a mx28 board, running "aplay -l" and "arecord -l" results in the following:

$ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: mxssgtl5000 [mxs_sgtl5000], device 0: Playback sgtl5000-0 []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: mxssgtl5000 [mxs_sgtl5000], device 1: Capture sgtl5000-1 []
  Subdevices: 1/1
  Subdevice #0: subdevice #0

$ arecord -l
**** List of CAPTURE Hardware Devices ****
card 0: mxssgtl5000 [mxs_sgtl5000], device 0: Playback sgtl5000-0 []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: mxssgtl5000 [mxs_sgtl5000], device 1: Capture sgtl5000-1 []
  Subdevices: 1/1
  Subdevice #0: subdevice #0

,which is not correct because we got a capture device listed in aplay and a
playback device listed in arecord.

On mx28 there are two serial audio interface ports (SAIF0 and SAIF1) and each
one of them are unidirectional.

Allow to specify a dai link as 'playback_only' or 'capture_only', which suits
well for this case.

After this change we can correctly report the capabilities as follows:

$ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: mxssgtl5000 [mxs_sgtl5000], device 0: HiFi Playback sgtl5000-0 []
  Subdevices: 1/1
  Subdevice #0: subdevice #0

$ arecord -l
**** List of CAPTURE Hardware Devices ****
card 0: mxssgtl5000 [mxs_sgtl5000], device 1: HiFi Capture sgtl5000-1 []
  Subdevices: 1/1
  Subdevice #0: subdevice #0

Also tested playback and capture on the mx28evk board.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

sound/soc/mxs/mxs-sgtl5000.c

@@ -91,11 +91,13 @@ static struct snd_soc_dai_link mxs_sgtl5000_dai[] = {
 		.stream_name	= "HiFi Playback",
 		.codec_dai_name	= "sgtl5000",
 		.ops		= &mxs_sgtl5000_hifi_ops,
+		.playback_only	= true,
 	}, {
 		.name		= "HiFi Rx",
 		.stream_name	= "HiFi Capture",
 		.codec_dai_name	= "sgtl5000",
 		.ops		= &mxs_sgtl5000_hifi_ops,
+		.capture_only	= true,
 	},
 };