[MTD] Make init_rpxlite function static

The function init_rpxlite, which is not used outside of
drivers/mtd/maps/rpxlite.c, 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>