api: storage: Fix enumeration of storage devices

dev_stor_get() is not able to find the next available device in the current
storage group when the previous enumerated device belongs to a different
group or class (e.g. network).

The root cause is the device group iterator not being reset after an
unsuccessful search for the last returned device so that the following
search for the next available device will start from beginning.

The issue has been identified by loading and booting GRUB in a QEMU
vexpress-a9 environment.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@iquestgroup.com>