ASoC: simple-card: Fix the compile warning.

sound/soc/generic/simple-card.c: In function simple_card_dai_link_of:
sound/soc/generic/simple-card.c:198:10: warning: passing argument 3 of
asoc_simple_card_sub_parse_of from incompatible pointer type [enabled by default]
          &dai_link->cpu_dai_name);
          ^
sound/soc/generic/simple-card.c:112:1: note: expected const struct device_node **
but argument is of type struct device_node **
 asoc_simple_card_sub_parse_of(struct device_node *np,
 ^
sound/soc/generic/simple-card.c:229:10: warning: passing argument 3 of
asoc_simple_card_sub_parse_of from incompatible pointer type [enabled by default]
          &dai_link->codec_dai_name);
          ^
sound/soc/generic/simple-card.c:112:1: note: expected const struct device_node **
but argument is of type struct device_node **
 asoc_simple_card_sub_parse_of(struct device_node *np,
 ^

Since the asoc_simple_card_sub_parse_of() is used in simple-card module only,
and the third argument is just used to get the node ponters address, so there is
no need it must to be 'const' type.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>