mmc: dw_mmc: return the result of mmc_add_host()

Check the result of mmc_add_host() and return the value.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>