api_storage: Fix non-first storage device enumeration

When enabling CONFIG_API and chain-loading GRUB2 on jetson-tk1, only the
eMMC would show up as (hd0), but not the SD card, leading to GRUB not
finding its configuration and modules, falling back to a rescue shell.

This is because enum_ended would get set for !more after returning a
cookie for the first MMC device in group 3.

Fix this by properly setting the "more" argument also in the case of the
first storage device of a group.

Signed-off-by: Andreas Färber <afaerber@suse.de>