crypto: stm32/hash - fix performance issues

Fixing bugs link to stress tests. Bad results are
detected during testmgr selftests executing in a
faster environment. bufcnt value may be resetted and
false IT are sometimes detected.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>