crypto: ixp4xx - Simplify and harden key parsing

Use the common helper function crypto_authenc_extractkeys() for key
parsing. Also ensure the keys do fit into the corresponding buffers.
Otherwise memory corruption might occur.

Cc: Christian Hohnstaedt <chohnstaedt@innominate.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Mathias Krause <mathias.krause@secunet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>