usbip: only dump valid port status

The wIndex parameter of vhci_hub_control() is always zero when the
request type is GetHubDescriptor, see drivers/usb/core/hub.c::get_hub_descriptor()
and Universal Serial Bus Specification revision 2.0 (April 27, 2000) Section
11.24.2.5 "Get Hub Descriptor". This means the variable rhport will not contain
any useful index. Only use valid rhport values.

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>