crypto: stm32/hash - Fix hmac issue more than 256 bytes

Correct condition for the second hmac loop. Key must be only
set in the first loop. Initial condition was wrong,
HMAC_KEY flag was not properly checked.

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