mmc: sdhci-xenon: use match_string() helper

match_string() returns the index of an array for a matching string,
which can be used intead of open coded variant.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Hu Ziji <huziji@marvell.com>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: linux-mmc@vger.kernel.org
Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>