USB: Make sure to fetch the BOS desc for roothubs.

The BOS descriptor is normally fetched and stored in the usb_device->bos
during enumeration.  USB 3.0 roothubs don't undergo enumeration, but we
need them to have a BOS descriptor, since each xHCI host has a different
U1 and U2 exit latency.  Make sure to fetch the BOS descriptor for USB
3.0 roothubs.  It will be freed when the roothub usb_device is released.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: Andiry Xu <andiry.xu@amd.com>