debugfs: constify argument to debugfs_real_fops()

seq_file users can only access const version of file pointer,
because the ->file member of struct seq_operations is marked
as such.  Make parameter to debugfs_real_fops() const.

CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
CC: Nicolai Stange <nicstange@gmail.com>
CC: Christian Lamparter <chunkeey@gmail.com>
CC: LKML <linux-kernel@vger.kernel.org>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>