sfc (gcc13): synchronize ef100_enqueue_skb()'s return type

ef100_enqueue_skb() generates a valid warning with gcc-13:
  drivers/net/ethernet/sfc/ef100_tx.c:370:5: error: conflicting types for 'ef100_enqueue_skb' due to enum/integer mismatch; have 'int(struct efx_tx_queue *, struct sk_buff *)'
  drivers/net/ethernet/sfc/ef100_tx.h:25:13: note: previous declaration of 'ef100_enqueue_skb' with type 'netdev_tx_t(struct efx_tx_queue *, struct sk_buff *)'

I.e. the type of the ef100_enqueue_skb()'s return value in the declaration is
int, while the definition spells enum netdev_tx_t. Synchronize them to the
latter.

Cc: Martin Liska <mliska@suse.cz>
Cc: Edward Cree <ecree.xilinx@gmail.com>
Cc: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20221031114440.10461-1-jirislaby@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>