mmc: dw_mmc: improve dw_mci_reset a bit

Too much condition iteration makes the code
less readable. Slightly improve it.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>