disk: fix blk_get_device_part_str()

blk_get_device_part_str() should always initialize all info fields
including sys_ind. As a side effect the code is simplified.

Replace '(0 ==' by '(!' to conform with Linux coding style.

Fixes: 4d907025 ("sandbox: restore ability to access host fs through standard commands")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Heiko Schocher <hs@denx.de>