mmc: sdhci: Don't signal the sdio irq if it's not setup

[Not needed in newer kernels due to refactoring fixing this issue.]

With 3.14.29 (and older kernels) some of my I.mx6 Sabrelite boards were
crashing with the following oops:

  sdhci: Secure Digital Host Controller Interface driver
  sdhci: Copyright(c) Pierre Ossman
  sdhci-pltfm: SDHCI platform and OF driver helper
  sdhci-esdhc-imx 2198000.usdhc: could not get ultra high speed state, work on normal mode
  mmc0: no vqmmc regulator found
  mmc0: SDHCI controller on 2198000.usdhc [2198000.usdhc] using ADMA
  Unable to handle kernel NULL pointer dereference at virtual address 00000000
  pgd = c0004000
  [00000000] *pgd=00000000
  Internal error: Oops: 5 [#1] SMP ARM
  Modules linked in:
  CPU: 0 PID: 0 Comm: swapper/0 Not tainted 3.14.29 #1
  task: c08a7120 ti: c089c000 task.ti: c089c000
  PC is at wake_up_process+0x8/0x40
  LR is at sdhci_irq+0x748/0x9c4

Full boot log can be found at:
  http://storage.kernelci.org/stable/v3.14.29/arm-multi_v7_defconfig/lab-collabora/boot-imx6q-sabrelite.html

This happens if the sdhci interrupt status contains SDHCI_INT_CARD_INT,
while the sdio irq was never setup.  This patch fixes that in a minimal
way by checking if the sdio irq was setup.

In more recent kernels this bug went away due to refactoring done by
Russel King. So an alternative (potentially better?) fix for this patch
is to cherrypick the following patches from a recent kernel:

18258f72 - genirq: Provide synchronize_hardirq()
bf3b5ec6 - mmc: sdio_irq: rework sdio irq handling
41005003 - mmc: sdhci: clean up interrupt handling
781e989c - mmc: sdhci: convert to new SDIO IRQ handling
Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Tyler Baker <tyler.baker@linaro.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

drivers/mmc/host/sdhci.c

@@ -2508,7 +2508,7 @@ static irqreturn_t sdhci_irq(int irq, void *dev_id)
 	/*
 	 * We have to delay this as it calls back into the driver.
 	 */
-	if (cardint)
+	if (cardint && host->mmc->sdio_irqs)
 		mmc_signal_sdio_irq(host->mmc);
 
 	return result;