drivers: counter: Add __fallthrough in STM32 counter driver

STM32 counter driver uses switch statement in which cases don't end
with break or return intentionally.

Affected switches in counter driver check status of all timer channels
(maximum 4 channels), but the number of channels is not determined
during compilation. In switch, we jump to channel with highest number
and then check other channels with lower numbers.

Compiler can warn about it, so this patch adds information that it was
intentional.

Signed-off-by: Patryk Duda <pdk@semihalf.com>