target: cortex_a: add break in switch/case

The code falls-through in the default case, making it not easy to
read.

Add the explicit break to improve the readability.

Change-Id: I4784b883e0e82258de17018dfdfb59b4042ac743
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/9049
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

src/target/cortex_a.c

@@ -2360,6 +2360,7 @@ static int cortex_a_write_cpu_memory(struct target *target,
 					count *= 2;
 					size = 2;
 				}
+				break;
 			case 0:
 			default:
 				break;