mmc: dw_mmc: clear IDSTS register when initialize IDMAC

If pending interrupt for IDMAC exists when initialize IDMAC, it will
call interrupt handler unnecessarily.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>