crypto: caam - simplify and harden key parsing

Use the common helper function crypto_authenc_extractkeys() for key
parsing.
Also fix the key buffer overflow condition: use split key pad length
instead of authentication key length.

Signed-off-by: Horia Geanta <horia.geanta@freescale.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>