USB: xhci: Set Mult field in endpoint context correctly.

The bmAttributes field of the SuperSpeed Endpoint Companion Descriptor has
different meanings, depending on the endpoint type.  If the endpoint is
isochronous, the bmAttributes field is the maximum number of packets
within a service interval that this endpoint supports.  If the endpoint is
bulk, it's the number of stream IDs this endpoint supports.

Only set the Mult field of the xHCI endpoint context using the
bmAttributes field if the endpoint is isochronous, and the device is a
SuperSpeed device.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>