drm: fix writing to /sys/class/drm/*/status

Writing to a file is supposed to return the number of bytes written.
Returning zero unfortunately causes bash to constantly spin trying
to write to the sysfs file, to such an extent that even ^c and ^z
have no effect.  The only way out of that is to kill the shell and
log back in.  This isn't nice behaviour.

Fix it by returning the number of characters written to sysfs files.

[airlied: used suggestion from Al Viro]
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>