Bluetooth: BAP: UC: Fix call to bt_gatt_get_mtu in notify

In the unicast_client_ep_notify we would always call
bt_gatt_get_mtu, regardless of whether data == NULL.

When there is a disconnection, the notify callbacks are
called with data == NULL to indicate a unsubscription.
In the case of the unicast client, this would also trigger
calls to bt_gatt_get_mtu when there is a disconnect, which
in turn would trigger a warning that ATT is not connected.

Postponing the call to bt_gatt_get_mtu fixes this.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>