cmd: pinmux: Fix warnings when compiling with W=1

This patch solves the following warnings:

cmd/pinmux.c: In function 'do_dev':
cmd/pinmux.c:26:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (ret) {
      ^
cmd/pinmux.c:30:2: note: here
  case 1:
  ^~~~
Signed-off-by: NPatrice Chotard <patrice.chotard@st.com>
Signed-off-by: NPatrick Delaunay <patrick.delaunay@st.com>

cmd/pinmux.c

@@ -27,6 +27,7 @@ static int do_dev(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 			printf("Can't get the pin-controller: %s!\n", name);
 			return CMD_RET_FAILURE;
 		}
+		/* fall through */
 	case 1:
 		if (!currdev) {
 			printf("Pin-controller device is not set!\n");