staging: comedi: dt9812: pass the comedi_device * to dt9812_configure_gain()

For aesthetic reasons, instead of passing the struct usb_dt9812 pointer,
pass the comedi_device pointer to this function.

Tidy up the function and use the comedi_device class_dev for the
dev_err() message.

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>