hwrng: stm32 - rename readl return value

Use a more meaningful name for the readl return value variable.

Link: https://lore.kernel.org/all/Y1J3QwynPFIlfrIv@loth.rohan.me.apana.org.au/



Signed-off-by: Tomas Marek <tomas.marek@elrest.cz>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>