BTA_GATTC_PrepareWrite() function

* Function BTA_GATTC_PrepareWrite * Description This function is called to prepare write a characteristic value. * Parameters conn_id - connection ID. * p_char_id - GATT characteristic ID of the service. * offset - offset of the write value. * len: length of the data to be written. * p_value - the value to be written. * Returns None