KEYS: asymmetric: Fix error codes

These error paths should return the appropriate error codes instead of
returning success.

Fixes: 63ba4d67

 ("KEYS: asymmetric: Use new crypto interface without scatterlists")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>