Check input properly in backlight, echo > brightness should not turn off
the backlight.

[akpm@linux-foundation.org: coding-style fixes]
[akpm@linux-foundation.org: fix printk warning]
Signed-off-by: NPavel Machek <pavel@suse.cz>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NRichard Purdie <rpurdie@linux.intel.com>