iio: adc: stm32-adc: set some stm32-adc.c variables storage-class-specifier to static

iio: adc: stm32-adc: set some stm32-adc.c variables storage-class-specifier to static

smatch reports several warnings
drivers/iio/adc/stm32-adc.c:2591:20: warning:
  symbol 'stm32_adc_min_ts_h7' was not declared. Should it be static?
drivers/iio/adc/stm32-adc.c:2610:20: warning:
  symbol 'stm32_adc_min_ts_mp1' was not declared. Should it be static?
drivers/iio/adc/stm32-adc.c:2630:20: warning:
  symbol 'stm32_adc_min_ts_mp13' was not declared. Should it be static?

These variables are only used in stm32-adc.c, so they should be static

Signed-off-by: Tom Rix <trix@redhat.com>
Link: https://lore.kernel.org/r/20230312161733.470617-1-trix@redhat.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>