mmc: dw_mmc: remove unused member variable.

Since using the device-tree, didn't use the callback pointer.
So removed the unused callback pointer.
When the set_power callback is used, it should be added in future.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>