sfc: remove duplicate declaration of efx_enqueue_skb_tso()

Define it in nic_common.h, even though the ef100 driver will have a
 different implementation backing it (actually a WARN_ON_ONCE as it
 should never get called by ef100.  But it needs to still exist because
 common TX path code references it).

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>