SI: Don't cancel transfer with TSTART=0

[Bonta0]

As far as I can tell there is no such thing as transfer cancelling, I don't know where that came from. This hasn't been an issue so far because Dolphin's transfer are instantaneous except for GBAs
This was confirmed on hardware with

printf("start csr %08X buf %08X%08X\n", *si_comcsr, si_buffer[0], si_buffer[1]);
si_buffer[0] = 0; *si_comcsr = 0x40010303;
printf("started csr %08X buf %08X%08X\n", *si_comcsr, si_buffer[0], si_buffer[1]);
*si_comcsr &= ~SICOMCSR_TSTART;
printf("cancelled csr %08X buf %08X%08X\n", *si_comcsr, si_buffer[0], si_buffer[1]);
while(*si_comcsr & SICOMCSR_TSTART);
printf("waited csr %08X buf %08X%08X\n", *si_comcsr, si_buffer[0], si_buffer[1]);

[JMC47]

Fixes Pokemon Colosseum hanging with a GBA connected on boot.

[booto]

Looks okay to me - nice that the code is simpler with this. If you're feeling motivated, a test somewhere in https://github.com/dolphin-emu/hwtests exercising the behavior could be great, but thanks for the contribution all the same!

[JMC47]

I've tested this and verified the behavior on both GameCube and Wii. I ran the aforementioned tests on Wii, with a variety of controller configurations in order to verify the behavior. LGTM.