greybus: sdio: add switch voltage operation

Core sd/mmc needs the start_signal_voltage_switch operation to be
defined to issue a voltage switch command. So, we define it here even
though we do not need to take in action on it, since all voltage control
is done in the Module.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>