mmc: omap: convert to per instance workqueue

Currently, a global mmc_omap_wq is created for all instances of omap
hosts, which can lead to races and doesn't lend itself to unload the
module cleanly.  Instead, create per instance workqueue and remove
 the common workqueue.

Signed-off-by: Venkatraman S <svenkatr@ti.com>
Acked-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>