Comparing both functions, it seems that nas_config_mbms_s1() sets the
same parameters, assuming that in nas_config_mbms(), we set
- (UE_NAS_USE_TUN || ENB_NAS_USE_TUN) yielding true -> should yield the
  same interface name
- netmask is hardcoded to 255.255.255.0
- baseNetAddress is hardcoded 10.0

So replace the call with nas_config_mbms() by setting parameters
accordingly.