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

Fix the following warnings when documentation is built:
drivers/counter/stm32-lptimer-cnt.c:354: warning: cannot understand
function prototype: 'enum stm32_lptim_cnt_function'

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Fixes: 597f55e3 ("counter: stm32-lptimer: add counter device")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>