mmc: rtsx: remove driving adjustment

Several new models of readers use different way to select driving
capability (a necessary adjustment along with voltage change). Removing
this from device-independent rtsx_pci_sdmmc module. It will be implemented
in device-depend calls encapsulated by rtsx_pci_switch_output_voltage().

Signed-off-by: Roger Tseng <rogerable@realtek.com>
Reviewed-by: Wei WANG <wei_wang@realsil.com.cn>
Signed-off-by: Chris Ball <cjb@laptop.org>