usbnet: Resubmit interrupt URB if device is open

Resubmit interrupt URB if device is open.  Use a flag set in
usbnet_open() to determine this state.  Also kill and free
interrupt URB in usbnet_disconnect().

[Rebased off git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git

]

Signed-off-by: Paul Stewart <pstew@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>