hwrng: stm32 - use sizeof(*priv) instead of sizeof(struct stm32_rng_private)

Use sizeof(*priv) instead of sizeof(struct stm32_rng_private), the
former makes renaming of struct stm32_rng_private easier if necessary,
as it removes one site where such rename has to happen. No functional
change.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Uwe Kleine-König <ukleinek@kernel.org>
Acked-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>