Fix const correctness of EC_KEY_METHOD_get_*

Changes the EC_KEY_METHOD_get_* family to not need a EC_KEY_METHOD* as
its first parameter, but a const EC_KEY_METHOD*, which is entirely
sufficient.

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
GH: #3985
(cherry picked from commit 4e9b720e)