ASoC: stm32: sai: remove set but not used variables 'mask, cr1'

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

sound/soc/stm/stm32_sai_sub.c: In function 'stm32_sai_configure_clock':
sound/soc/stm/stm32_sai_sub.c:902:11: warning:
 variable 'mask' set but not used [-Wunused-but-set-variable]
sound/soc/stm/stm32_sai_sub.c:902:6: warning:
 variable 'cr1' set but not used [-Wunused-but-set-variable]

It's not used any more after 8307b2af ("ASoC: stm32: sai: set sai as
mclk clock provider")

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