mmc: core: Break out start_signal_voltage_switch

Allow callers to access the start_signal_voltage_switch host_ops
member without going through any cmd11 logic. This is mostly a
preparation for the following signal voltage switch patch.

Also, reset ios.signal_voltage to its original value if
start_signal_voltage_switch fails.

Signed-off-by: Johan Rudholm <johan.rudholm@stericsson.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Wei WANG <wei_wang@realsil.com.cn>
Signed-off-by: Chris Ball <cjb@laptop.org>