crypto: stm32/cryp - remove request mutex protection

Mutex is badly used between threaded irq and driver.
This mutex must be removed as the framework must ensure
that requests must be serialized to avoid issue. Rework
req to avoid crash during finalize by fixing the NULL
pointer issue.

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