drivers: entropy: stm32: don't use zero value

According to the Reference Manual of several series(G0,L5,WL,WB,...)
RNG_DR register value should only be used if it is different from 0:
"Because when it is the case a seed error occurred between RNG_SR
polling and RND_DR output reading (rare event)."

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>