ASoC: imx-audmux: remove null check of audmux_base in audmux_read_file

When audmux_read_file is called, it means the driver is already
initialised successfully, so we don't need to check audmux_base.

It also fix smatch warning:
sound/soc/fsl/imx-audmux.c:78 audmux_read_file() warn: possible memory leak of 'buf'

Signed-off-by: Richard Zhao <richard.zhao@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>