[MTD] Make init_sbc82xx_flash function static

The function init_sbc82xx_flash is needlessly defined global in
drivers/mtd/maps/wr_sbc82xx_flash.c, and this patch makes it static.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.fi>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>