api: storage: Test all block device in dev_stor_get

In a config with one MMC at device id '1' and no MMC at device id '0'
(a BeagleBone Black with no sd inserted for example), the current code
will first test to access the MMC 0 (sd port), seeing that no device is
present it will simply return that no more device are present for this
class.
This patch fixes this by testing all devices for each class.

Signed-off-by: Emmanuel Vadot <manu@bidouilliste.com>