USB: dummy-hcd: remove unsupported isochronous endpoints

The dummy-hcd driver doesn't support emulation of isochronous
transfers.  Therefore it doesn't need to export isochronous endpoint
descriptors; they can be commented out.

Also, the comments in the source code don't express clearly enough the
fact that isochronous isn't supported.  They need to be more explicit.

Finally, change the error status value we use (in theory) for
isochronous URBs.  checkpatch complains about ENOSYS; EINVAL is more
appropriate (it is documented to mean "ISO madness").

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>