crypto: chtls - key len correction

corrected the key length to copy 128b key. Removed 192b and 256b
key as user input supports key of size 128b in gcm_ctx

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Atul Gupta <atul.gupta@chelsio.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>