mmc: mediatek: Remove redundant runtime PM calls

Commit 9250aea7 ("mmc: core: Enable runtime PM management of host
devices"), made some calls to the runtime PM API from the driver
redundant. Especially those which deals with runtime PM reference
counting, so let's remove them.

Cc: Chaotian Jing <chaotian.jing@mediatek.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>