HID: cougar: Stop processing vendor events on hid-core

Special key events received by the custom vendor's hdev are
translated to key events on the kbd iface's input device, so
their processing must not continue. Return -EPERM from
raw_event handler to effectively stop source events from
being processed in hid-core.

Signed-off-by: Daniel M. Lambea <dmlambea@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>