USB Network driver infrastructure: Fix leak when usb_autopm_get_interface() returns less than zero in kevent().

We'll leak the memory allocated to 'urb' in
drivers/net/usb/usbnet.c:kevent() when we 'goto fail_lowmem' and the 'urb'
variable goes out of scope while still completely unused.
Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: David S. Miller <davem@davemloft.net>