mmc: omap_hsmmc: Introduce omap_hsmmc_prepare/complete

prepare() is supposed to prevent new children from being registered.
On the MMC subsystem, children (new cards) registration starts with
the card detect IRQ.

Move card detect IRQ disabling to prepare() so that no new cards
will be registered while we're trying to suspend.

Likewise, move card detect IRQ enabling to complete() so we only
try to register new children after our MMC IP is back up.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Venkatraman S <svenkatr@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>