mmc: dw_mmc: rework the code related to cmd/data completion

Main change corresponds to dw_mci_command_complete().  And EBE is
divided into read and write.  Some minor changes for code readability.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Tested-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>