omap_hsmmc: only MMC1 allows HCTL.SDVS != 1.8V

Based on a patch from Tony Lindgren ... after initialization,
never change HCTL.SDVS except for MMC1.  The other controller
instances only support 1.8V in that field, although they can
suport other card/SDIO/eMMC/... voltages with level shifting
solutions such as external transceivers.

MMC2 behavior sanity tested on Overo/WLAN, OMAP3430 SDP, and
custom hardware.  MMC1 also sanity tested on those platforms
plus Beagle.  This also fixes a bug preventing MMC2 (and also
presumably MMC3) from powering down when requested.

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