I'm not a big DSP guy, but I think you can probably expect questions about your background in ASIC or FPGA design; what tools you've used; what devices you've targeted; VHDL/Verilog knowledge; what algorithms you've implemented or are familiar with; how you would do debugging. You might expect a question about a specific problem, for example data compression, and how you'd approach it.
Basically, I don't think you can 'study' for an interview other than knowing what the specific company does and what they are looking for. Either you know your stuff or you don't. If you're interviewing for a position that involves video processing you might want to review 2D image processing, but I wouldn't worry about control systems.