HID: core: replace the collection tree pointers with indices

Previously, the pointer to the parent collection was stored. If a device
exceeds 16 collections (HID_DEFAULT_NUM_COLLECTIONS), the array to store
the collections is reallocated, the pointer to the parent collection becomes
invalid.

Replace the pointers with an index-based lookup into the collections array.

Fixes: c53431eb

 ("HID: core: store the collections as a basic tree")
Reported-by: Pandruvada, Srinivas <srinivas.pandruvada@intel.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Tested-by: Kyle Pelton <kyle.d.pelton@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>