cmd: convert fallthrough comment.

cmd: convert fallthrough comment.

gcc does not understand /* FALL TROUGH */ and emits a warning:

    cmd/date.c: In function ‘do_date’:
    cmd/date.c:62:20: warning:
    this statement may fall through [-Wimplicit-fallthrough=]
       62 |                 if (strcmp(argv[1],"reset") == 0) {
          |                    ^
    cmd/date.c:102:9: note: here
      102 |         case 1:                 /* get date & time */
          |         ^~~~

Use the fallthrough macro instead.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>