sfc: Move TX queue core queue mapping into tx.c

efx_hard_start_xmit() needs to implement a mapping which is the
inverse of tx_queue::core_txq.  Move the initialisation of
tx_queue::core_txq next to efx_hard_start_xmit() to make the
connection more obvious.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>