drivers: timer: stm32_lptim: fix sys_clock_* return value

This commit finish to fix the bug describe by 85e2a0679a68f02f7ef.
With the previous correction, the uptime read could be in the past:
if the counter rewinds just after testing ARRM flag, we had lost
some counts.

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