Change signature of openid_cp_field_get_mapping()

openid_cp_field_get_mapping($mapping_sreg, $type .'_sreg');

This is not the best way to go.

openid_cp_field_get_mapping(&$mapping, $type = FALSE, $ext = 'ax');
This should be the function signature.
After this change::
openid_cp_field_get_mapping($mapping_sreg, $type, 'sreg');

Do it after #444170: Hook changes and client side SReg support only, conflicting patchset.
Looks good?