can: etas_es58x: rewrite the message cast in es58{1,_fd}_tx_can_msg to increase readability

The es581_tx_can_msg() does below cast:
|	tx_can_msg = (struct es581_4_tx_can_msg *)
|	    &es581_4_urb_cmd->bulk_tx_can_msg.tx_can_msg_buf[msg_len - 1];

The -1 offset is to skip the first member of struct
es581_4_tx_can_msg. However, es581_4_urb_cmd already has a raw_msg
field which can be used instead without the use of an offset
calculation.

The function es58x_fd_tx_can_msg() is also modified to mirror
es581_tx_can_msg().

Link: https://lore.kernel.org/r/20210628155420.1176217-7-mailhol.vincent@wanadoo.frSigned-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/usb/etas_es58x/es581_4.c

@@ -371,8 +371,7 @@ static int es581_4_tx_can_msg(struct es58x_priv *priv,
 		return ret;
 
 	/* Fill message contents. */
-	tx_can_msg = (struct es581_4_tx_can_msg *)
-	    &es581_4_urb_cmd->bulk_tx_can_msg.tx_can_msg_buf[msg_len - 1];
+	tx_can_msg = (typeof(tx_can_msg))&es581_4_urb_cmd->raw_msg[msg_len];
 	put_unaligned_le32(es58x_get_raw_can_id(cf), &tx_can_msg->can_id);
 	put_unaligned_le32(priv->tx_head, &tx_can_msg->packet_idx);
 	put_unaligned_le16((u16)es58x_get_flags(skb), &tx_can_msg->flags);

drivers/net/can/usb/etas_es58x/es58x_fd.c

@@ -357,8 +357,7 @@ static int es58x_fd_tx_can_msg(struct es58x_priv *priv,
 		return ret;
 
 	/* Fill message contents. */
-	tx_can_msg = (struct es58x_fd_tx_can_msg *)
-	    &es58x_fd_urb_cmd->tx_can_msg_buf[msg_len];
+	tx_can_msg = (typeof(tx_can_msg))&es58x_fd_urb_cmd->raw_msg[msg_len];
 	tx_can_msg->packet_idx = (u8)priv->tx_head;
 	put_unaligned_le32(es58x_get_raw_can_id(cf), &tx_can_msg->can_id);
 	tx_can_msg->flags = (u8)es58x_get_flags(skb);