crypto: stm32/cryp - fix xts and race condition in crypto_engine requests

Don't erase key:
If key is erased before the crypto_finalize_.*_request() call, some
pending process will run with a key={ 0 }.
Moreover if the key is reset at end of request, it breaks xts chaining
mode, as for last xts block (in case input len is not a multiple of
block) a new AES request is started without calling again set_key().

Fixes: 9e054ec2 ("crypto: stm32 - Support for STM32 CRYP crypto module")

Signed-off-by: Nicolas Toromanoff <nicolas.toromanoff@foss.st.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>