fastboot: Keep zircon_r successful in set_active

`fastboot set_active` marks all other slot invalid including zircon_r.
This breaks `dm reboot recovery` and `fastboot recovery`.

BUG=b/222580161
TEST=Tested with `dm reboot recovery` and `fastboot reboot recovery`
BRANCH=fuchsia/atlas

Change-Id: Id96f7475014888ca5cb079d7f3b0e9b20efac19f
Signed-off-by: Yecheng Zhao <zyecheng@google.com>
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/depthcharge/+/654062
Reviewed-by: David Pursell <dpursell@google.com>