SSO: ensure that 'return to your site' link goes to correct page

#### Steps to reproduce the issue
1. Connect with Jetpack, turn on SSO, and add the `jetpack_sso_bypass_login_forward_wpcom` filter using:

``` php
add_filter( 'jetpack_sso_bypass_login_forward_wpcom', '__return_true' );
```
1. Make sure you are are logged in to WordPress.com with any account
2. Go to the login page for your site at yoursite.com/wp-admin
3. Click on the 'Return to Your Site' link at the bottom of the sign in screen
4. See that you are redirected back to yoursite.com/wp-admin instead of yoursite.com
It looks like nothing is happening because it's the same page as before.
#### What I expected

I expected to be redirected back to yoursite.com