ASoC: dapm: Fix possible uninitialized variable in snd_soc_dapm_get_volsw()

If soc_dapm_read() fails, val will be uninitialized, and bogus values
will be written later:

	ret = soc_dapm_read(dapm, reg, &val);
	val = (val >> shift) & mask;

However, the compiler does not give a warning. Return on error before
val is really used to avoid this.

This is similar to the commit 69128316 ("ASoC: dapm: Fix
uninitialized variable in snd_soc_dapm_get_enum_double()")

Fixes: ce0fc93a (ASoC: Add DAPM support at the component level)
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Mark Brown <broonie@kernel.org>