Staging: dgnc: Fix struct file_operations should normally be const

This patch fixes following checkpatch.pl warning:
WARNING:struct file_operations should normally be const

Signed-off-by: Monam Agarwal <monamagarwal123@gmail.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>