mmc: omap_hsmmc: remove unused .set_sleep function

set_sleep seems to be unused in omap_hsmmc driver. so get rid of it.

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