mmc: sdhci: Reduce spin lock usage in sdhci_execute_tuning

mmc: sdhci: Reduce spin lock usage in sdhci_execute_tuning

Tuning execution is already synchronized with respect to other host
operations by upper layers "claiming" the host, which also takes care of
runtime pm. There can be no requests in progress. Retain the spin lock
usage only for ensuring that sending tuning commands is synchronized with
respect to the interrupt handler.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Ludovic Desroches <ludovic.desroches@microchip.com>