clocksource/drivers/stm32: Fix all kernel-doc warnings

Add a "Returns:" section in one function description.
Use the correct function name in another function description.

These changes prevent 2 warnings:

timer-stm32.c:79: warning: No description found for return value of 'stm32_timer_of_bits_get'
timer-stm32.c:189: warning: expecting prototype for stm32_timer_width(). Prototype was for stm32_timer_set_width() instead

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Cc: linux-stm32@st-md-mailman.stormreply.com
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20240120173615.14618-1-rdunlap@infradead.org