Ah Pai didn't look at your code, but I remembered that Cypress EZ-USB FX2 has the 24MHz crystal that you mentioned, but it has also a multiplier to run the FX2 at frequencies up to 48MHz (12MHz, 24MHz and 48MHz). So even if the crystal is 24MHz the FX2 might run at 12MHz. If you take a look at the technical manual of the FX2 mcu, you 'll see that the responsible bits for setting the frequency are bits 3 and 4 of the CPUCS (CPU Control and Status) SFR register. At power-on-reset, these bits default to 00 (12 MHz).
So your code may be correct, but you haven't checked the frequency.