USB: EHCI: adjust error return code

The USB stack uses error code -ENOSPC to indicate that the periodic
schedule is too full, with insufficient bandwidth to accommodate a new
allocation.  It uses -EFBIG to indicate that an isochronous transfer
could not be linked into the schedule because it would exceed the
number of isochronous packets the host controller driver can handle
(generally because the new transfer would extend too far into the
future).

ehci-hcd uses the wrong error code at one point.  This patch fixes it,
along with a misleading comment and debugging message.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>