mtd: sa1100: Mark expected switch fall-through

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 <[e-mail]>

drivers/mtd/maps/sa1100-flash.c

···
 	default:
 		printk(KERN_WARNING "SA1100 flash: unknown base address "
 		       "0x%08lx, assuming CS0\n", phys);
+		/* Fall through */
 
 	case SA1100_CS0_PHYS:
 		subdev->map.bankwidth = (MSC0 & MSC_RBW) ? 2 : 4;