CRC-CCITT
@param - uint8_t *ptr pointer to the data in a array
@param - int count the length of the data array
@param - uint16_t initCRC the initial value of CRC16, the header comes with a recommanded predefine initial value
@return the calculated crc16 value
check your result at: http://www.sunshine2k.de/coding/javascript/crc/crc_js.html