gatt: Fix infinite loop on ACL disconnect

Fix obvious mistake.
Error can be visible if multiple BLE devices is in used, some of the
GATT commands are queued and then first disconnected device has no queue
GATT commands.

Bug: 257046947
Test: manual
 - Connect BLE devices X, Y
 - Queue multiple commands on device X
 - Disconnect device Y

Tag: #feature
Merged-In: Ic0877bf7322dfa5bf0103bb6eab18cadac51244b
Change-Id: Ic0877bf7322dfa5bf0103bb6eab18cadac51244b