mmc: sdhci: Use work structs instead of tasklets

mmc: sdhci: Use work structs instead of tasklets

The driver can happily live without an atomic context and tasklets,
so turn the tasklets into the work structs.

Tasklets handlers still grab irqsave spinlocks, but we'll deal
with it in a separate 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>