crypto: stm32/hash - Use existing busy poll function

When exporting state we are waiting indefinitely in the same
was as the ordinary stm32_hash_wait_busy() poll-for-completion
function but without a timeout, which means we could hang in
an eternal loop. Fix this by waiting for completion like the
rest of the code.

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>