mmc: sdhci: Get rid of host->busy_handle

Now that there is host->data_cmd to record the command for which a data
interrupt is expected, it is possible to determine whether a command with
busy signaling has completed without an extra flag. So host->busy_handle
is not needed. Remove it.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>