[MTD] Make lart_flash_init, lart_flash_exit static

The symbols lart_flash_init, lart_flash_exit are needlessly
defined global in drivers/mtd/devices/lart.c, so make them
static.

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