staging: comedi: ni_mio_common: make more bits in ao_cmd1 reg be stateful

Bits NISTC_AO_CMD1_DAC0_UPDATE_MODE and NISTC_AO_CMD1_DAC1_UPDATE_MODE are
now saved in the local copy of the AO_CMD1 register.  This is more
appropriate than prior methods of setting these bits specifically _both_
in the ni_ao_cmd configuration function _and_ the ni_ao_inttrig trigger
function.  With this patch, the bits are only specifically called out now
in the ni_ao_cmd configuration function.  In the ni_ao_inttrig trigger
function, only the UI_ARM, UC_ARM, BC_ARM bits of the ao_cmd1 register are
specifically called out.  Each of these bits is a strobe bit, while the
DAC[0-1]_UPDATE_MODE bits are simple write bits.

Signed-off-by: Spencer E. Olson <olsonse@umich.edu>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>