crypto: chelsio - Use library to prepare HMAC keys

To prepare HMAC keys, just use the library functions instead of
crypto_shash.  This is much simpler, avoids depending on the fragile
export_core and import_core methods, and is faster too.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>