shared/gatt-client: Fix sending confirmation

Confirmation shall be sent from topmost parent only.
Extra confirmation sent by clone confuses remote side.

> ACL Data RX: Handle 69 flags 0x02 dlen 27
      ATT: Handle Value Indication (0x1d) len 22
        Handle: 0x001f
          Data: 68656c6c6f2027302e302e302e303a3532353332
< ACL Data TX: Handle 69 flags 0x00 dlen 5
      ATT: Handle Value Confirmation (0x1e) len 0
< ACL Data TX: Handle 69 flags 0x00 dlen 5
      ATT: Handle Value Confirmation (0x1e) len 0
1 file changed