mmc: sdhci: Tidy sdhci_request() a bit

In preparation for further changes, tidy sdhci_request() a bit.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Baolin Wang <baolin.wang7@gmail.com>
Link: https://lore.kernel.org/r/20200412090349.1607-5-adrian.hunter@intel.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>