ASoC: fsl: Fix memory leak in imx-audmux.c

When audmux_clk is used and clk_prepare_enable function succeed,
the memory alloc'd to buf variable is leaked

Signed-off-by: Felipe Pena <felipensp@gmail.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>