mmc-uclass: correct the device number

Not like the mmc-legacy which the devnum starts from 1, it starts from 0
in mmc-uclass, so the device number should be (devnum + 1) in get_mmc_num().

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>