pflash_cfi01: Fix warning caused by unreachable code

Report from smatch:
hw/pflash_cfi01.c:431 pflash_write(180) info: ignoring unreachable code.

Instead of removing the return statement after the switch statement,
the patch replaces the return statements in the switch statement by
break statements. Other switch statements in the same code do it also
like that.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NStefan Hajnoczi <stefanha@gmail.com>

hw/pflash_cfi01.c

@@ -321,7 +321,7 @@ static void pflash_write(pflash_t *pfl, target_phys_addr_t offset,
         }
         pfl->wcycle++;
         pfl->cmd = cmd;
-        return;
+        break;
     case 1:
         switch (pfl->cmd) {
         case 0x10: /* Single Byte Program */
@@ -376,7 +376,7 @@ static void pflash_write(pflash_t *pfl, target_phys_addr_t offset,
         default:
             goto error_flash;
         }
-        return;
+        break;
     case 2:
         switch (pfl->cmd) {
         case 0xe8: /* Block write */
@@ -407,7 +407,7 @@ static void pflash_write(pflash_t *pfl, target_phys_addr_t offset,
         default:
             goto error_flash;
         }
-        return;
+        break;
     case 3: /* Confirm mode */
         switch (pfl->cmd) {
         case 0xe8: /* Block write */
@@ -423,7 +423,7 @@ static void pflash_write(pflash_t *pfl, target_phys_addr_t offset,
         default:
             goto error_flash;
         }
-        return;
+        break;
     default:
         /* Should never happen */
         DPRINTF("%s: invalid write state\n",  __func__);