mmc: host: omap_hsmmc: Remove setting PBIAS voltage

PBIAS voltage should be set along with setting vqmmc voltage and
these voltages should be set as part of start_signal_voltage_switch
callback. However since omap_hsmmc is about to be deprecated,
remove setting of PBIAS voltage leaving the PBIAS voltage to be
at the reset value of 3.3V (we'll never have to change this to 1.8V
since UHS mode support will not be added to omap_hsmmc). This will
let pbias regulator driver to be fixed to support a maximum voltage of
3.3V.

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