Handle the possibility that SDHCI_PLATFORM_START_TRANSFER() can fail, by
moving the handling of curcmd->error != 0 to the end of the interrupt
handler. Also make sdhci_finish_data() idempotent by moving the setting
of slot->data_done = 1 down past the point where the busdma buffer is
unmapped. This allows for the possibility that the finish routine can
get called from multiple places when handling errors.