HID: input mapping for Chicony KU-0418 tactical pad

Chicony KU-0418 (aka Saitek PZ08AU gaming keyboard) has a separate
"tactical pad" with 11 non-functional buttons - they generate usage
codes from 0xff00 (MSVENDOR) usage page. Special case handling for
this keyboard added, so no later clash with MSVENDOR mappings is going
to occur.

Pointed out in bugzilla #7352

Signed-off-by: Jiri Kosina <jkosina@suse.cz>