mmc: tmio: Share register access functions

Move register access functions into a shared header.
Use sd_ctrl_write16 in tmio_mmc_dma.c:tmio_mmc_enable_dma().

Other than avoiding (trivial) open-coding, the motivation for
this is to allow platform-hooks in access functions to
be applied across all applicable accesses.

Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: Magnus Damm <magnus.damm@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Chris Ball <cjb@laptop.org>
diff --git a/drivers/mmc/host/tmio_mmc_dma.c b/drivers/mmc/host/tmio_mmc_dma.c
index 9c4da66..f24a029 100644
--- a/drivers/mmc/host/tmio_mmc_dma.c
+++ b/drivers/mmc/host/tmio_mmc_dma.c
@@ -26,7 +26,7 @@
 {
 #if defined(CONFIG_SUPERH) || defined(CONFIG_ARCH_SHMOBILE)
 	/* Switch DMA mode on or off - SuperH specific? */
-	writew(enable ? 2 : 0, host->ctl + (CTL_DMA_ENABLE << host->bus_shift));
+	sd_ctrl_write16(host, enable ? 2 : 0, CTL_DMA_ENABLE);
 #endif
 }