sdio_uart: Use kfifo instead of the messy circ stuff

Revised patch to use the new kfifo API. This replaces the one that was dropped
from -next due to collisions with the kfifo API changes.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>