counter: stm32-timer-cnt: probe number of channels from registers

counter: stm32-timer-cnt: probe number of channels from registers

Probe the number of capture compare channels, by writing CCER register bits
and read them back. Take care to restore the register original value.

This is a precursor patch to support capture channels.

Reviewed-by: William Breathitt Gray <william.gray@linaro.org>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20240307133306.383045-9-fabrice.gasnier@foss.st.com
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>