Using opaque keys for static ECDH is not supported in this branch (will
be introduced in 3.2). In case we reach that point, error out cleanly
instead of miscasting a pointer. Since opaque keys were introduced,
mbedtls_pk_can_do() was no longer a precise enough check.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>