drivers/sensor: lps22hh: Fix int32 overflow in the val2 part

The val2 calculation was done using (1000000 / 40960) as
multiplying factor, which was sometimes leading to a
int32 overflow. So, let's use the equivalent (but smaller)
(3125 / 128).

Fix #38090

Signed-off-by: Armando Visconti <armando.visconti@st.com>