ASoC: rt5677: Refactor code to avoid comparison unsigned >= 0

rt5677_to_irq() has couple of redundant conditionals, one of which
compares unsigned variable to be great than or equal to zero which
is always true as compiler notices:

sound/soc/codecs/rt5677.c: In function ‘rt5677_to_irq’:
sound/soc/codecs/rt5677.c:4626:13: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]

Refactor the code by removing redundant conditionals.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>