crypto: ecdsa - Fix the public key format description

Public key blob is not just x and y concatenated. It follows RFC5480
section 2.2. Address this by re-documenting the function with the
correct description of the format.

Link: https://datatracker.ietf.org/doc/html/rfc5480
Fixes: 4e660291

 ("crypto: ecdsa - Add support for ECDSA signature verification")
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>