cx88: fix setting input when using DVB

In cx88-mpeg.c, there is code that sets core->input to CX88_VMUX_DVB. However, this may be incorrect, since core->input is actually an index to core->board.input[], which has not enough elements to be indexed by the value of CX88_VMUX_DVB. So, the modified code searches core->board.input[] for an input with a type of CX88_VMUX_DVB, and if it does not find one, the index is simply set to zero.
The change may not have much effect, though, since it appears the only case when core->input is actually used is when the current input is being queried.