Bluetooth: gatt: avoid assumptions about characteristic handles

Although the Characteristic Value descriptor is required to be
immediately after the characteristic descriptor, the specification
allows for gaps in the corresponding Attribute handles.  Use the value
handle from the characteristic descriptor for value reads.

See BLUETOOTH CORE SPECIFICATION Version 5.1 Vol 3, Part G section 2.5.1
(p. 2345), first paragraph.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>