crypto: tcrypt - fix keysize argument of test_aead_speed for gcm(aes)

The key sizes used by AES in GCM mode should be 128, 192 or 256 bits (16,
24 or 32 bytes).
There is no additional 4byte nonce as for RFC 4106.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>