Fix change-of-sign warning detected by ICC

subscribeIndex is quint16, so it can't store a value of -1. Store 65535
instead.

Note: in findgeneric, subIdx will NOT be -1. It will be 65535.

Change-Id: I4b5ad6415609f409660a035de6f763ddb2da2f7e
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>