[MTD] Make init_msp_flash function static

The function init_msp_flash, which is not used outside of
drivers/mtd/maps/pmcmsp-flash.c, can become static. This
patch adds the needed keyword.

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