staging: comedi: ni_mio_common: move ni_stc_dma_channel_select_bitfield()

Move this inline helper function from ni_stc.h. It does not need to be
exposed outside of this file.

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>