ASoC: imx-audmux: Fix section mismatch

audmux_debugfs_init() is marked as __init, but is called from imx_audmux_probe()
which is not marked as __init. This creates a section mismatch and a potential
runtime crash (if imx_audmux_probe() is called after the .init section was
dropped). This patch removes the __init annotation from audmux_debugfs_init(),
which fixes the following warning:
	WARNING: sound/soc/built-in.o(.text+0x86960): Section mismatch in reference
	from the function imx_audmux_probe() to the function
	.init.text:audmux_debugfs_init()

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>