mmc: sdhci-xenon: Fix the work flow in xenon_remove().

sdhci_remove_host() might execute SOFT_RESET_ALL.  Inside xenon_remove(),
Xenon SDHC should be enabled during sdhci_remove_host().

Move xenon_sdhc_unprepare after sdhci_remove_host() such that Xenon SDHC is
disabled after sdhci_remove_host() completes.

Signed-off-by: Hu Ziji <huziji@marvell.com>
Reported-by: Jisheng Zhang <jszhang@marvell.com>
Tested-by: Jisheng Zhang <jszhang@marvell.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>