LE: Add GATT disable functions

This patch adds required disable functions to the GATT sub-system to
properly unregister with the stack. Without the disable functions in
place, turning Bluetooth off with a GATT device connected may lead to
unexpected behaviour and cause GATT to fail on sub-sequent stack
restarts.

Change-Id: I7cb80e96109e2c09882991298d0487b506f5ffdd