Since sec->key_sizes[] is an u8, len can't be < 0.

Spotted by the Coverity checker.
Signed-off-by: Adrian Bunk <bunk@stusta.de>
Cc: "John W. Linville" <linville@tuxdriver.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>