iio: adc: stm32: make array stm32h7_adc_ckmodes_spec static

The array stm32h7_adc_ckmodes_spec does not need to be in global scope, so
make it static.

Cleans up sparse warning:
"symbol 'stm32h7_adc_ckmodes_spec' was not declared. Should it be static?"

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>