crypto: stm32/hash - Wait for idle before final CPU xmit

When calculating the hash using the CPU, right before the final
hash calculation, heavy testing on Ux500 reveals that it is wise
to wait for the hardware to go idle before calculating the
final hash.

The default test vectors mostly worked fine, but when I used the
extensive tests and stress the hardware I ran into this problem.

Acked-by: Lionel Debieve <lionel.debieve@foss.st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>