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

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

This patch solves the following warnings:

drivers/adc/stm32-adc.c: In function 'stm32_adc_chan_of_init':
warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if (num_channels > adc->cfg->max_channels) {
                   ^
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>