staging: dgnc: check the type of print before calling

The dgnc_maxcps_room() function must be called only for print device.
The if-statement for checking print device checks before calling
dgnc_maxcps_room() and also this function doesn't need to have any
data except channel_t.

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>