gattc: Fix uninitialized memory issue which could lead to crash

This is regression after
6e5f0111 gattc: Extend gattc queue with mtu exchange

This patch makes sure that new mtu_cb and mtu_cb_data
are set to 0 when not used.

Tag: #feature
Test: CtsVerifier
Sponsor: jpawlowski@
Fixes: 178129488
Bug: 178129488
Change-Id: Ia550c99ee7292fa275150791fc73e92ae791d91c