sx.c: fix dbl statement if - add missing braces

Caused by 736d5453 (sx.c: fix missed unlock_kernel() on error path in
sx_fw_ioctl()).  You guys keep breaking things this way in every single
kernel release in at least couple of places...  :-(

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Acked-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>