gatt: Fix gatt_sr_is_cback_cnt_zero

This check is meant to be done per CID.

Bug: 385287242
Test: mmm packages/modules/Bluetooth
Test: manual test
1. create multiple EATT channels
2. Trigger Read Request on all the ATT bearers
3. Check all the responses arrives

Flag: Exempt, day-to-day fix
Change-Id: Idc8ca8a0023616a0add83a7c2df7ff1ce2fcc7ce