OMAP: mmc_twl4030 nicely disable vmmc_aux

The MMC driver turns the power off before it turns it
on. To avoid regulator warnings, vmmc_aux must only be
disabled if it has previously been enabled.

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>