counter: stm32-timer-cnt: fix a kernel-doc warning

Fix the following warning when documentation is built:
drivers/counter/stm32-timer-cnt.c:37: warning: cannot understand function
prototype: 'enum stm32_count_function'

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Fixes: ad29937e ("counter: Add STM32 Timer quadrature encoder")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>