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.fr


Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>