mmc: host: omap_hsmmc: enable/disable vmmc_aux regulator based on previous state

enable vmmc_aux regulator only if it is in disabled state and disable
vmmc_aux regulator only if it is in enabled state.

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>