CYAPA_FW_HDR_SIZE
csum = cyapa_gen3_csum(&fw->data[2], CYAPA_FW_HDR_SIZE - 2);
csum_expected = (fw->data[CYAPA_FW_HDR_SIZE - 2] << 8) |
fw->data[CYAPA_FW_HDR_SIZE - 1];
csum = cyapa_gen3_csum(&fw->data[CYAPA_FW_HDR_SIZE],
#define CYAPA_FW_SIZE (CYAPA_FW_HDR_SIZE + CYAPA_FW_DATA_SIZE)