mmc: dw_mmc: remove the prepare_command hook

This patch removes the prepare_command hook from entire dw_mmc driver.
Now, almost all SoCs are using by default, except Exynos.
It seems that dwmmc controller is using unnecessary hook.
To know whether needs to set this bit or not,
add the DW_MMC_CARD_NO_USE_HOLD bit.

If some SoCs need to disable this in future, just set the
DW_MMC_CARD_NO_USE_HOLD bit.
set_bit(DW_MMC_CARD_NO_USE_HOLD, &slot->flags),

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