usb3503: clarify what the registers 'PDS' and 'CFG1' really do

The current comment sounds like you have to disable some of
the ports to be able to use self-powered mode. This is
misleading, so change the wording to reflect this.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>