ASoC: dapm: Fix uninitialized variable in snd_soc_dapm_get_enum_double()

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

sound/soc/soc-dapm.c: In function 'snd_soc_dapm_get_enum_double':
sound/soc/soc-dapm.c:2862:15: warning: 'reg_val' may be used uninitialized in this function [-Wmaybe-uninitialized]
  unsigned int reg_val, val;
               ^

Return early on error to fix this.

Introduced by commit ce0fc93a ("ASoC:
Add DAPM support at the component level").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>