Misc: Fix doxygen comment for MaxPower field with respect to super-speed

Misc: Fix doxygen comment for MaxPower field with respect to super-speed

The MaxPower field of a configuration descriptor is in units of 8 mA
when the device is operating in super-speed mode according to the USB
3.0 specification, section 9.6.3, table 9-12:

  Maximum power consumption of the device from the
  bus in this specific configuration when the device is
  fully operational. Expressed in 2-mA units when the
  device is operating in high-speed mode and in 8-mA
  units when operating at Gen X speed.
  (i.e., 50 = 100 mA when operating at high-speed and
  50 = 400 mA when operating at Gen X speed).

Adjust the doxygen comment for the MaxPower member of struct
libusb_config_descriptor accordingly.

Closes #69

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>