Staging: dgnc: Fix checking return value of register_chrdev

The failure code is negative. So check <0 instead of <=0.
Return the failure code instead of -ENXIO.

Signed-off-by: Salah Triki <salah.triki@acm.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>