mmc: msm_sdcc: no voltage switch required if io voltage is low

MMC host ops "start_signal_voltage_switch" is called to switch the
vdd_io voltage level from high voltage to low voltage but if the vdd_io
voltage level is already set to low voltage range then there is no need
to perform the voltage switching. This patch ensures the same.

Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>