crypto: ecdh - fix typo of P-192 b value

Fix the b value to be compliant with FIPS 186-4 D.1.2.1. This fix is
required to make sure the SP800-56A public key test passes for P-192.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>