staging: comedi: ni_mio_common: use CR_CHAN more consistently

Generally, the CR_CHAN macro is/should be used to access the relevant bits
for channel identification in cmd->*_arg when the corresponding
cmd->*_src==TRIG_EXT, including cmd->convert_arg in this case.

This patch does not fix a bug per se, as NISTC_AI_MODE1_CONVERT_SRC() already
masks the value sufficiently, but using CR_CHAN() here makes the code clearer as
it avoids passing some irrelevant bits to NISTC_AI_MODE1_CONVERT_SRC() in the
first place.

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>