USB: gadget: dummy_hcd: fix hub-descriptor removable fields

commit d81182ce30dbd497a1e7047d7fda2af040347790 upstream.

Flag the first and only port as removable while also leaving the
remaining bits (including the reserved bit zero) unset in accordance
with the specifications:

	"Within a byte, if no port exists for a given location, the bit
	field representing the port characteristics shall be 0."

Also add a comment marking the legacy PortPwrCtrlMask field.

Fixes: 1cd8fd28 ("usb: gadget: dummy_hcd: add SuperSpeed support")
Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Cc: Tatyana Brokhman <tlinder@codeaurora.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>