sfc: farch: fix TX queue lookup in TX event handling

We're starting from a TXQ label, not a TXQ type, so
 efx_channel_get_tx_queue() is inappropriate (and could return NULL,
 leading to panics).

Fixes: 12804793 ("sfc: decouple TXQ type from label")
Cc: stable@vger.kernel.org
Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>