staging: comedi: ni_labpc: use labpc_have_dma_chan()

Call the new static inline function `labpc_have_dma_chan()` from
`labpc_ai_cmd()` to check if the ISA DMA channel has been initialized,
tidying up the surrounding code and removing an `#ifdef`.  If the
"ni_labpc_isadma" module is not being built, `labpc_have_dma_chan()`
doesn't bother checking the DMA channel and just returns `false`,
allowing the compiler to optimize out a small amount of code.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>