usb: ehci: fix memory leak in ehci_init_transfer

In ehci_init_transfer function, if the 'cpage' is bigger than 4,
it doesn't free the 'p->sgl' once allocated previously thus leading
a memory leak issue. This patch avoid this.

Signed-off-by: Li Qiang <liqiang6-s@360.cn>
Message-id: 5821c0f4.091c6b0a.e0c92.e811@mx.google.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
