USB: EHCI: No SSPLIT allowed in uframe 7

The scheduling code in ehci-hcd contains an error.  For full-speed
isochronous-OUT transfers, the EHCI spec forbids scheduling
Start-Split transactions in H-microframe 7, but the driver allows it
anyway.  This patch adds a check to prevent it.

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