odroid: adjust get_dfu_alt_*() functions to new declarations

This change is required after updated dfu_alt_system/boot declarations.

Signed-off-by: Inha Song <ideal.song@samsung.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>
[Test HW: Odroid U3 (Exynos 4412)]
Acked-by: Minkyu Kang <mk7.kang@samsung.com>