boot: bootflow_menu: fix crash for EFI BOOTMGR global bootmeth

The global bootmeths don't set the dev in bootflow struct which means
the dev_get_parent(bflow->dev) triggers a NULL-pointer dereference and
crash U-Boot.

So before trying to handle a bootflow, check that the associated
bootmeth isn't global, otherwise skip it.

Suggested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Simon Glass <sjg@chromium.org>