[MTD] Make init_fortunet function static

The symbol init_fortunet is needlessly defined global in
drivers/mtd/maps/fortunet.c. Make it static and clean up
the kernel global namespace.

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