mtd: omap2: fix static declaration warning

This patch fixes sparse warning for static declaration of variable "use_dma"

drivers/mtd/nand/omap2.c:114:11: warning: symbol 'use_dma' was not declared. Should it be static?

Signed-off-by: G, Manjunath Kondaiah <manjugk@ti.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>