crypto: stm32 - remove bufcnt in stm32_hash_write_ctrl.

Commit "crypto: stm32 - Fix empty message processing" remove the use of
the argument bufcnt in stm32_hash_write_ctrl.
Hence, we can remove it from the function prototype and simplify the
function declaration.

Signed-off-by: Thomas Bourgoin <thomas.bourgoin@foss.st.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>