crypto: bcm - convert to use crypto_authenc_extractkeys()

Convert the bcm crypto driver to use crypto_authenc_extractkeys() so
that it picks up the fix for broken validation of rtattr::rta_len.

This also fixes the DES weak key check to actually be done on the right
key. (It was checking the authentication key, not the encryption key...)

Fixes: 9d12ba86

 ("crypto: brcm - Add Broadcom SPU driver")
Cc: <stable@vger.kernel.org> # v4.11+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>