counter: stm32-timer-cnt: Add const qualifier for functions_list array

counter: stm32-timer-cnt: Add const qualifier for functions_list array

The struct counter_count functions_list member expects a const enum
counter_count_function array. This patch adds the const qualifier to the
stm32_count_functions to match functions_list.

Cc: Benjamin Gaignard <benjamin.gaignard@st.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Link: https://lore.kernel.org/r/46a1e7096dd9280d8f241894186b3c903956a55f.1623201081.git.vilhelm.gray@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>