ni_labpc: correct differential channel sequence for AI commands

ni_labpc: correct differential channel sequence for AI commands

Tuomas <tvainikk _at_ gmail _dot_ com> reported problems getting
meaningful output from a Lab-PC+ in differential mode for AI cmds, but
AI insn reads gave correct readings.  He tracked it down to two
problems, one of which is addressed by this patch.

It seems the setting of the channel bits for particular scanning modes
was incorrect for differential mode.  (Only half the number of channels
are available in differential mode; comedi refers to them as channels
0, 1, 2 and 3, but the hardware documentation refers to them as channels
0, 2, 4 and 6.)  In differential mode, the setting of the channel enable
bits in the command1 register should depend on whether the scan enable
bit is set.  Effectively, we need to double the comedi channel number
when the scan enable bit is not set in differential mode.  The scan
enable bit gets set when the AI scan mode is MODE_MULT_CHAN_UP or
MODE_MULT_CHAN_DOWN, and gets cleared when the AI scan mode is
MODE_SINGLE_CHAN or MODE_SINGLE_CHAN_INTERVAL.  The existing test for
whether the comedi channel number needs to be doubled in differential
mode is incorrect in labpc_ai_cmd().  This patch corrects the test.

Thanks to Tuomas for suggesting the fix.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>