CALL CRC16_INIT
CALL CRC16_ADD
CALL CRC16_GET

Before making call to CRC16_ADD you need to put the data byte in Accumulator for instance your data byte is 'A' thus your code should look like

CALL CRC16_INIT
MOV A,#'A'
CALL CRC16_ADD
CALL CRC16_GET

The other mistake you are making is the code you posted is for CRC_CCITT and your CRC is initialised as 0x0000 where as the page you are talking about initialises your CRC as 0xffff.

Regards,
Prahlad Purohit