ASoC: tlv320aic32x4: Remove set but not used variable 'mclk_rate'

Fixes gcc '-Wunused-but-set-variable' warning:

sound/soc/codecs/tlv320aic32x4.c: In function 'aic32x4_setup_clocks':
sound/soc/codecs/tlv320aic32x4.c:669:16: warning: variable 'mclk_rate' set but not used [-Wunused-but-set-variable]

It is not used since introduction in
commit 96c3bb00 ("ASoC: tlv320aic32x4: Dynamically Determine Clocking")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>