Implements the backend so that the generic driver can TX/RX to/from
the SDIO device.

For RX, when data is ready the SDIO IRQ is fired and that will
allocate an skb, put all the data there and then pass it to the
generic driver RX code for processing and delivery.

TX, when kicked by the generic driver, will schedule work on a
driver-specific workqueue that pulls data from the TX FIFO and sends
it to the device until it drains it.

Thread contexts are needed as SDIO functions are blocking.
Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>