misdn: use nonseekable_open()

The driver just sets ->llseek to NULL.  It should also clear FMODE_LSEEK to
tell the VFS that seeks are not supported.

Pointed out by Christoph Hellwig.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>