mmc: host: omap_hsmmc: add separate function to set pbias

No functional change. Cleanup omap_hsmmc_set_power by adding separate
functions to set pbias and invoke it from omap_hsmmc_set_power.

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>