staging: comedi: dt9812: use CR_CHAN() for channel number

As pointed out by Dan Carpenper in
<http://driverdev.linuxdriverproject.org/pipermail/devel/2013-February/036025.html

>,
the dt9812 comedi driver's use of the `chanspec` member of `struct
comedi_insn` as a channel number is incorrect.  Change it to use
`CR_CHAN(insn->chanspec)` as the channel number (where `insn` is a
pointer to the `struct comedi_insn` being processed).

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable <stable@vger.kernel.org> # 3.8 onwards
Cc: Anders Blomdell <anders.blomdell@control.lth.se>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>