crypto: ecrdsa - Fix signature size calculation

software_key_query() returns the curve size as maximum signature size
for ecrdsa.  However it should return twice as much.

It's only the maximum signature size that seems to be off.  The maximum
digest size is likewise set to the curve size, but that's correct as it
matches the checks in ecrdsa_set_pub_key() and ecrdsa_verify().

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>