typedef enum {
  LM75A_REG_TEMP = 0,      /*!< Temperature Register   */                                 
  LM75A_REG_CFG  = 1,      /*!< Configuration Register */                                 
  LM75A_REG_THYS = 2,      /*!< Hysteresis Register    */
  LM75A_REG_TOS  = 3,      /*!< Over-Limit Signal Temperature register */
} LM75A_REG;

uint16_t LM75A_ReadReg (LM75A_REG reg) {
  switch(reg) {
.......   
  case LM75A_REG_TEMP: /* Temperature Register */
    cfg.rx_length = 2; break;

  case LM75A_REG_CFG:  /* Configuration Register */
    cfg.rx_length = 1; break;

  case LM75A_REG_TOS:  /* Hysteresis Register */
    cfg.rx_length = 2; break;

  case LM75A_REG_THYS: /* Over-Limit Signal Temperature register */
    cfg.rx_length = 2; break;

  default :       break;
  }
........   
}

  temp_hex = LM75A_ReadReg(LM75A_REG_TEMP);