staging: comedi: ni_mio_common: tidy up mite dma channel releases

Absorb the inline helper functions that call ni_set_bitfield() to clear
the dma channel selection bits.

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>