[meshcop-tlv] fix ChannelMaskTlv::GetChannelMask() (#5947)
[meshcop-tlv] fix ChannelMaskTlv::GetChannelMask() (#5947)


jwhui/openthread,abtink/openthread,openthread/openthread,jwhui/openthread,srickardti/openthread,openthread/openthread,abtink/openthread,abtink/openthread,openthread/openthread,srickardti/openthread,srickardti/openthread,jwhui/openthread,srickardti/openthread,jwhui/openthread,openthread/openthread,abtink/openthread