mmc: dw_mmc: display the real register value on debugfs

Developer wants to see the real register value, not register offset.
This patch fixed to display the real value of register.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>