stm32mp1: rng: remove superfluous assignment

We should not assign a value that is overwritten before use.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>