drivers: timer: stm32 lptimer when sleeping for -1

When CONFIG_SYSTEM_SLOPPY_IDLE is not set, then system
can sleep for -1 means waking Up at the max possible
counter value (INT_MAX)
When CONFIG_SYSTEM_SLOPPY_IDLE is set sleeping K_TICKS_FOREVER
means never wakingUp

Signed-off-by: Francois Ramu <francois.ramu@st.com>