MAINTAINERS: mmc: Remove Seungwon Jeon from dw_mmc

Unfortunate, Seungwon's email has been bouncing for some time and it seems
like he has moved on to other duties. So, I would like to thank him for
his efforts, but it's now time to remove him as the co-maintainer for the
dw_mmc driver.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>