ARM: OMAP3: mmc-twl4030 fix for vmmc = 0

Resolve longstanding issue noted by Adrian Hunter:  confusion
between settting VSEL=0 (which is 1.8V on MMC1) and poweroff.

Also, leave VSEL alone if we're just powering the regulator off.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>