mmc: dw_mmc: make multiple instances of dw_mci_card_workqueue

The variable 'dw_mci_card_workqueue' is a global variable shared between
multiple instances of the dw_mmc host controller. Due to this, data
corruption has been noticed when multiple instances of dw_mmc controllers
are actively reading/writing the media. Fix this by adding a instance
of 'struct workqueue_struct' for each host instance and removing the
global 'dw_mci_card_workqueue' instance.

Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>