[MTD] Make init_mbx function static

The function init_mbx can become static, because it is not used
outside the file drivers/mtd/maps/mbx860.c. This patch adds the
needed keyword.

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