driver: timer: stm32_lptim: fix excess ticks

Some ticks are counted additionally when the autoreload
interrupts were too close together.
This patch improve the counts of the clock cycle.
lptim_fired worked badly in particular because the flag ARRM
was not raised when the interrupt was forced.

Signed-off-by: Julien D'Ascenzio <julien.dascenzio@paratronic.fr>