register: refactor register_cache_invalidate()

register_cache_invalidate() is written a way which uses
pointer arithmetic, which makes it harder to read. This patch
replaces it with more readable way to iterate over array of
structs.

Change-Id: Ia420f70a3bb6998c690c8c600c71301dca9f9dbf
Signed-off-by: Marek Vrbka <marek.vrbka@codasip.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7735
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins
Reviewed-by: Jan Matyas <jan.matyas@codasip.com>

src/target/register.c

@@ -93,9 +93,8 @@ void register_unlink_cache(struct reg_cache **cache_p, const struct reg_cache *c
 /** Marks the contents of the register cache as invalid (and clean). */
 void register_cache_invalidate(struct reg_cache *cache)
 {
-	struct reg *reg = cache->reg_list;
-
-	for (unsigned int n = cache->num_regs; n != 0; n--, reg++) {
+	for (unsigned int n = 0; n < cache->num_regs; n++) {
+		struct reg *reg = &cache->reg_list[n];
 		if (!reg->exist)
 			continue;
 		reg->valid = false;