mmc: dw_mmc: fix compile error when CONFIG_MMC_DW_IDMAC is disabled

When disable CONFIG_MMC_DW_IDMAC, can see the compiler error.
Because in dw_mci_post_req(), called the dw_mci_get_dma_dir().
But that function is in #ifdef CONFIG_MMC_DW_IDMAC.

I think that function is generic function.
Not need the CONFIG_MMC_DW_IDMAC.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Acked-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>