staging: comedi: dt9812: rename 'comedi' variable in struct slot_dt9812

Because of the usb (*probe) and comedi (*attach) disconnect, the struct
slot_dt9812 is used to pass the device data between the usb_driver and
the comedi_driver. The variable 'comedi' in this struct is used during
the comedi (*attach) to indicate if a slot is currently being used.

For aesthetic reasons, rename the variable to 'devpriv' since that is
what is actually saved in the pointer.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>