rtl8187: Remove pointless check in rtl8187_rx_cb().

First of all, it exposes the SKB list implementation.

Second of all it's not needed.  If we get called here, we
successfully enqueued the URB with the linked SKB and
such a completion only gets called one time on such an
SKB.

Signed-off-by: David S. Miller <davem@davemloft.net>