mmc: host: omap_hsmmc: avoid pbias regulator enable on power off

Fix omap_hsmmc_set_power so that pbias regulator is not enabled
during power off.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>