usb: ehci: fix memory leak in ehci_process_itd

While processing isochronous transfer descriptors(iTD), if the page
select(PG) field value is out of bands it will return. In this
situation the ehci's sg list is not freed thus leading to a memory
leak issue. This patch avoid this.

Signed-off-by: Li Qiang <liqiang6-s@360.cn>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
