crypto: ecdh - make ecdh_shared_secret unique

There is another ecdh_shared_secret in net/bluetooth/ecc.c

Fixes: 3c4b2390 ("crypto: ecdh - Add ECDH software support")
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>