drivers/video/fbdev/sm501fb.c: avoid world-writable sysfs files.

In line with practice for module parameters, we're adding a build-time
check that sysfs files aren't world-writable.

Cc: Vincent Sanders <vince@simtec.co.uk>
Cc: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>