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>

sound/soc/fsl/imx-audmux.c

@@ -74,9 +74,6 @@ static ssize_t audmux_read_file(struct file *file, char __user *user_buf,
 	if (!buf)
 		return -ENOMEM;
 
-	if (!audmux_base)
-		return -ENOSYS;
-
 	if (audmux_clk)
 		clk_prepare_enable(audmux_clk);