Nik, my suggestion is to wait for the TI AM335x SDK version 07.00.00.00. The expected release date is the end of 2014Q1, which is in a day or so. The new SDK will be based on kernel version 3.12 and will definitely include the TI ADC driver, so porting it to the Angstrom kernel 3.8 would be a lot easier.
Angstrom is supported at the Beagle community and if you do a quick search there, perhaps you will find someone who has already ported this.
From a quick look at your code, it seems OK. Do you successfully retrieve the IRQ? Are you sure nothing else in your system uses this interrupt? If it does, make sure the IRQF_SHARED (I may be wrong about the name) flag is set.
Best regards,
Miroslav