gsm_fcs_add_block
*fcs = gsm_fcs_add_block(INIT_FCS, dp , msg->data - dp);
*fcs = gsm_fcs_add_block(*fcs, msg->data, msg->len);
gsm->fcs = gsm_fcs_add_block(gsm->fcs, gsm->buf,
gsm->fcs = gsm_fcs_add_block(gsm->fcs, gsm->buf,
*dp = 0xFF - gsm_fcs_add_block(INIT_FCS, msg->data, dp - msg->data);