shared/gatt-client: Fix gatt_client_init
shared/gatt-client: Fix gatt_client_init

This function is only called within bt_gatt_client_new therefore it is
not possible to have a ready callback and should return false if
bt_gatt_exchange_mtu fails.

silent-snowman/bluez,ComputeCycles/bluez,pkarasev3/bluez,mapfau/bluez,silent-snowman/bluez,pkarasev3/bluez,ComputeCycles/bluez,mapfau/bluez,mapfau/bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,mapfau/bluez,silent-snowman/bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,ComputeCycles/bluez