mmc: omap_hsmmc: fix host reference after mmc_free_host

struct omap_hsmmc_host *host should not be accessed after mmc_free_host().
Reorder mmc_free_host() after iounmap(host->base).

Signed-off-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Venkatraman S <svenkatr@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>