mmc: Check for device with a seq number equal to num before checking against index

First check if there is an alias for the device tree node defined with the
given num before checking against device index.

Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>