staging/dgnc: fix info leak in ioctl

The dgnc_mgmt_ioctl() code fails to initialize the 16 _reserved bytes of
struct digi_dinfo after the ->dinfo_nboards member. Add an explicit
memset(0) before filling the structure to avoid the info leak.

Signed-off-by: Salva Peiró <speirofr@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>