crypto: tcrypt - fix buffer lengths in test_aead_speed()

Fix the way the length of the buffers used for
encryption / decryption are computed.
For e.g. in case of encryption, input buffer does not contain
an authentication tag.

Signed-off-by: Robert Baronescu <robert.baronescu@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>