staging: make new character devices nonseekable

As a preparation for changing the default behaviour of llseek to no_llseek,
every file_operations structure should have a .llseek operation.

There are three new instances in staging now, which can all be changed
into no_llseek explicitly since the devices do not need to seek.

Add nonseekable_open where appropriate, to prevent pread/pwrite as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>