mmc: dw_mmc: fix multiple drv_data NULL dereferences

800d78bfccb3d ("mmc: dw_mmc: add support for implementation specific
callbacks") -- merged in v3.7-rc1 -- introduced multiple NULL pointer
dereferences when the default dw_mci_pltfm_probe() is used, as it sets
host->drv_data to NULL, and that's only checked against NULL in 1 out of
the 7 cases where it is dereferenced.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>