staging: comedi: ni_660x: cleanup the NI660X_DMA_CFG register helpers

The BUG_ON() checks in the helper functions are not necessary. The mite
driver quiries the PCI chip to determine the number of DMA channels.
This is then used when a DMA channel is requested so the channel will
always be in range.

Convert the inline functions used to set the bits in the NI600X_DMA_CFG
register into macros. Also convert the associated enum dma_selection.
This clarifies the association with the register.

Rename the associated 'dma_configuration_soft_copies' member of the
private data to allow shorting some of the ugly long lines in the
driver.

This also fixes a number of checkpatch.pl issues about:

WARNING: Prefer 'unsigned int' to bare use of 'unsigned'

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>