Fix the warnings when using the W=1 compiler flag.

sound/soc/codecs/tlv320adcx140.c: In function ‘adcx140_reset’:
sound/soc/codecs/tlv320adcx140.c:570:6: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]
  570 |  int ret = 0;
      |      ^~~

This was set but only used in case where the reset GPIO is not defined.
Have the function return the value of ret.

sound/soc/codecs/tlv320adcx140.c: In function ‘adcx140_codec_probe’:
sound/soc/codecs/tlv320adcx140.c:778:18: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
  778 |  if (bias_source < ADCX140_MIC_BIAS_VAL_VREF ||
      |                  ^
sound/soc/codecs/tlv320adcx140.c:789:18: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
  789 |  if (vref_source < ADCX140_MIC_BIAS_VREF_275V ||

This condition will not occur since if the dt property is not set then
the *_source variable is set to the default value.  So there is no way
that *_source can be less then 0.  Which is what each #define is set to.
The code just needs to make sure that the dt property is not out of the
upper bounds.
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Link: https://lore.kernel.org/r/20200526175247.15309-1-dmurphy@ti.comSigned-off-by: Mark Brown <broonie@kernel.org>