Bluetooth: Host: Fix not sending Service Changed indication

gatt_unregister() clears handles if those were auto-assigned by host.
This resulted in Service Changed indication not beding sent since
call to sc_indicate() pass already cleared handles.

This was affecting GATT/SR/GAS/BV-01-C and GATT/SR/GAS/BV-07-C test
cases.

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>