mmc: host: omap_hsmmc: cleanup omap_hsmmc_reg_get()

No functional change. Instead of using a local regulator variable
in omap_hsmmc_reg_get() for holding the return value of
devm_regulator_get_optional() and then assigning to omap_hsmmc_host
regulator members: vcc, vcc_aux and pbias, directly use the
omap_hsmmc_host regulator members.

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