video: fbdev: acornfb: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

Fix the following warning (Building: rpc_defconfig arm):

drivers/video/fbdev/acornfb.c: In function ‘acornfb_parse_dram’:
drivers/video/fbdev/acornfb.c:860:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
    size *= 1024;
    ~~~~~^~~~~~~
drivers/video/fbdev/acornfb.c:861:3: note: here
   case 'K':
   ^~~~
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

drivers/video/fbdev/acornfb.c

@@ -858,6 +858,7 @@ static void acornfb_parse_dram(char *opt)
 		case 'M':
 		case 'm':
 			size *= 1024;
+			/* Fall through */
 		case 'K':
 		case 'k':
 			size *= 1024;