staging: comedi: adl_pci9118: tidy up 'ai_cfg' in pci9118_ai_docmd_dma()

The 'ai_cfg' was already set to PCI9118_AI_CFG_PDTRG | PCI9118_AI_CFG_PETRG.
Don't bother setting those bits in pci9118_ai_docmd_dma().

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>