mmc: sdhci: Fix SDIO IRQ thread deadlock

Since commit c07a48c2 ("mmc: sdhci: Remove finish_tasklet"), the IRQ
thread might be used to complete requests, but the IRQ thread is also used
to process SDIO card interrupts. This can cause a deadlock when the SDIO
processing tries to access the card since that would also require the IRQ
thread. Change SDHCI to use sdio_signal_irq() to schedule a work item
instead. That also requires implementing the ->ack_sdio_irq() mmc host op.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Fixes: c07a48c2 ("mmc: sdhci: Remove finish_tasklet")
Reported-by: Brian Masney <masneyb@onstation.org>
Tested-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>