mmc: sdhci: Use threaded IRQ handler

mmc: sdhci: Use threaded IRQ handler

We only need atomic context to disable SDHCI interrupts, after that
we can run in a kernel thread.

Note that irq handler still grabs an irqsave spinlock, we'll deal
with it in a subsequent patch.

Patch based on: http://thread.gmane.org/gmane.linux.kernel.mmc/2579.

Signed-off-by: Anton Vorontsov <avorontsov@mvista.com>
Signed-off-by: Jeremie Samuel <jeremie.samuel.ext@parrot.com>
Signed-off-by: Chris Ball <cjb@laptop.org>