mmc: dw_mmc: Drop redundant call to ->card_event callback

The ->card_event callback isn't being used by any of the dw_mmc variants.
It's likely a leftover from an earlier change, hence let's just drop the
redundant call to it.

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