mmc: mmc-uclass: Use dev_seq() to read aliases node's index

Use dev_seq() to read aliases node's index and pass it as device number
for creating bulk device.

Suggested-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>