ubi: Fix warning static is not at beginning of declaration

Compiler generates following warning when kernel is built with W=1:

drivers/mtd/ubi/ubi.h:971:1: warning: ‘static’ is not at beginning
of declaration [-Wold-style-declaration]

This commit fixes this by correctly ordering keywords.

Signed-off-by: Rishi Gupta <gupt21@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>