ASoC: core: Use kasprintf instead of opencoding it

kasprintf calculates the size of the result string, allocates a buffer large
enough to hold the string and then performs the format string operation. There
are a couple of places in ASoC where these three steps are done by hand and
where kasprintf can be used instead.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>