[MTD] Make init_sbc8240_mtd function static

The function init_sbc8240_mtd is not called from outside the
module drivers/mtd/maps/sbc8240.c where it is defined, so it
can become static. Add the needed keyword.

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