[MTD] Make init_sharpsl symbol static

The function init_sharpsl is needlessly defined global.
Make it static by adding the needed keyword to the file
drivers/mtd/maps/sharpsl-flash.c, where the function is
defined.

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