tests/qtest: xlnx-canfd-test: Fix code coverity issues

tests/qtest: xlnx-canfd-test: Fix code coverity issues

Following are done to fix the coverity issues:
1. Change read_data to fix the CID 1512899: Out-of-bounds access (OVERRUN)
2. Fix match_rx_tx_data to fix CID 1512900: Logically dead code (DEADCODE)
3. Replace rand() in generate_random_data() with g_rand_int()

Signed-off-by: Vikram Garhwal <vikram.garhwal@amd.com>
Message-id: 20230628202758.16398-1-vikram.garhwal@amd.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>