mmc: host: omap_hsmmc: use regulator_is_enabled to find pbias status

Use regulator_is_enabled of pbias regulator to find pbias regulator
status instead of maintaining a custom bookkeeping
pbias_enabled variable.

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>