The core "comedi" module and the "mite" helper module for NI PCI devices
both have calls to `dma_alloc_coherent()` and `dma_free_coherent()`.
Those functions are only available if `CONFIG_HAS_DMA` is defined.
Apart from the "mite" module, the functions are only called for comedi
drivers that set `s->async_dma_dir` (where `s` is a pointer to a `struct
comedi_subdevice`) to anything other than `DMA_NONE`.
Change local helper functions `__comedi_buf_alloc()` and
`__comedi_buf_free()` to only call `dma_alloc_coherent()` and
`dma_free_coherent()` if `CONFIG_HAS_DMA` is defined.
Change the "Kconfig" to make the following configuration options depend
on `HAS_DMA`: