USB: iuu_phoenix: add support for changing VCC

You can now set the IUU reader to 3.3V VCC instead of 5V VCC, using the sysfs
parameter vcc_mode. Valid values are 3 and 5.

Signed-off-by: Olivier Bornet <Olivier.Bornet@puck.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>