adc: stm32: Fix warnings when compiling with W=1

adc: stm32: Fix warnings when compiling with W=1

This patch solves the following warnings:

drivers/adc/stm32-adc-core.c: In function 'stm32h7_adc_clk_sel':
warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (i = 0; i < ARRAY_SIZE(stm32h7_adc_ckmodes_spec); i++) {
                 ^
warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (i = 0; i < ARRAY_SIZE(stm32h7_adc_ckmodes_spec); i++) {
                ^
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Acked-by: Fabrice Gasnier <fabrice.gasnier@st.com>