mmc: dw_mmc: amend use of idmac sw reset

First, compiling warning along with previous change is removed.
[drivers/mmc/host/dw_mmc.c:1890:7: warning: unused variable 'ctrl']
And with the recommendation in manual, IDMAC software reset is followed
by dma-reset of the CTRL register in order to terminate the transfer.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Tested-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>