Mark switch cases where we are expecting to fall through.

Fix the following warning (Building: assabet_defconfig arm):

drivers/mtd/maps/sa1100-flash.c: In function ‘sa1100_probe_subdev’:
drivers/mtd/maps/sa1100-flash.c:82:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   printk(KERN_WARNING "SA1100 flash: unknown base address "
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          "0x%08lx, assuming CS0\n", phys);
          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/mtd/maps/sa1100-flash.c:85:2: note: here
  case SA1100_CS0_PHYS:
  ^~~~
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>