mmc: dw_mmc: fixed a section mismatch in dw_mci_init_slot

Fixed the below message.

WARNING: drivers/mmc/host/built-in.o(.text+0x5ee8): Section mismatch in reference from the function dw_mci_probe() to the function .init.text:dw_mci_init_slot()
The function dw_mci_probe() references
the function __init dw_mci_init_slot().
This is often because dw_mci_probe lacks a __init
annotation or the annotation of dw_mci_init_slot is wrong

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>