usb-host: use correct altsetting in usb_host_ep_update

usb-host: use correct altsetting in usb_host_ep_update

In order to keep track of the alternate setting that should be used for
a given interface, the USBDevice struct keeps an array of alternate
setting values, which is indexed by the interface number. In
usb_host_set_interface, when this array is updated, usb_host_ep_update
is called as a result. However, when usb_host_ep_update accesses the
active libusb_config_descriptor, it indexes udev->altsetting with the
loop variable, rather than the interface number.

With the simple trace backend enable, this behavior can be seen:

  [...]

  usb_xhci_xfer_start 0.440 pid=1215 xfer=0x5596a4b85930 slotid=0x1 epid=0x1 streamid=0x0
  usb_packet_state_change 1.703 pid=1215 bus=0x1 port=b'1' ep=0x0 p=0x5596a4b85938 o=b'undef' n=b'setup'
  usb_host_req_control 2.269 pid=1215 bus=0x1 addr=0x5 p=0x5596a4b85938 req=0x10b value=0x1 index=0xd
  usb_host_set_interface 0.449 pid=1215 bus=0x1 addr=0x5 interface=0xd alt=0x1
  usb_host_parse_config 2542.648 pid=1215 bus=0x1 addr=0x5 value=0x2 active=0x1
  usb_host_parse_interface 1.804 pid=1215 bus=0x1 addr=0x5 num=0xc alt=0x0 active=0x1
  usb_host_parse_endpoint 2.012 pid=1215 bus=0x1 addr=0x5 ep=0x2 dir=b'in' type=b'int' active=0x1
  usb_host_parse_interface 1.598 pid=1215 bus=0x1 addr=0x5 num=0xd alt=0x0 active=0x1
  usb_host_req_emulated 3.593 pid=1215 bus=0x1 addr=0x5 p=0x5596a4b85938 status=0x0
  usb_packet_state_change 2.550 pid=1215 bus=0x1 port=b'1' ep=0x0 p=0x5596a4b85938 o=b'setup' n=b'complete'
  usb_xhci_xfer_success 4.298 pid=1215 xfer=0x5596a4b85930 bytes=0x0

  [...]

In particular, it is seen that although usb_host_set_interface sets the
alternate setting of interface 0xd to 0x1, usb_host_ep_update uses 0x0
as the alternate setting due to using the incorrect index to
udev->altsetting.

Fix this problem by getting the interface number from the active
libusb_config_descriptor, and then using that as the index to
udev->altsetting.

Signed-off-by: Nick Rosbrook <rosbrookn@ainfosec.com>
Message-Id: <20210201213021.500277-1-rosbrookn@ainfosec.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>