staging: comedi: ni_mio_common: introduce NI_STC_DMA_CHAN_SEL()

The inline helper ni_stc_dma_channel_select_bitfield() returns the
bits needed to select a MITE channel to use for DMA. The MITE code
is setup to handle up to 8 channels but in reality only channels 0
to 3 are used by most of the drivers. The PCI-6032E and PCI-6033E
boards can also use channels 4 and 5.

For aesthetics, convert this inline function into a macro and
remove the BUG() which will never occur.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>