non-checkout payment methods with assigned adjustment appear in checkout page

Issue Summary

problem:
- if a payment method is flagged as method, which is not available in checkout (eg. payment method "other") and despite this a adjustment amount is set for the method (in form 'uc_payment_methods_form'), the method appears inclomplete in checkout page of the order. All information except adjustment amount is missing.

proposal:
- when changing 'uc_cart_checkout_form' in hook_form_alter(), beside check for
.....variable_get('uc_payment_method_'. $method['id'] .'_adjustment', '');
a check for $method['checkout'] (or even the corresponding storage in $conf-array) has to be processed for adding any information to form.