can: etas_es58x: use sizeof and sizeof_field macros instead of constant values

Replace two constant values by a call to sizeof{,_field} on the
relevant field to make the logic easier to understand.

Link: https://lore.kernel.org/r/20210628155420.1176217-6-mailhol.vincent@wanadoo.fr
Signed-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

@@ -355,7 +355,7 @@ static int es581_4_tx_can_msg(struct es58x_priv *priv,
 		return -EMSGSIZE;
 
 	if (priv->tx_can_msg_cnt == 0) {
-		msg_len = 1; /* struct es581_4_bulk_tx_can_msg:num_can_msg */
+		msg_len = sizeof(es581_4_urb_cmd->bulk_tx_can_msg.num_can_msg);
 		es581_4_fill_urb_header(urb_cmd, ES581_4_CAN_COMMAND_TYPE,
 					ES581_4_CMD_ID_TX_MSG,
 					priv->channel_idx, msg_len);

drivers/net/can/usb/etas_es58x/es58x_core.c

@@ -70,7 +70,7 @@ MODULE_DEVICE_TABLE(usb, es58x_id_table);
  * bytes (the start of frame) are skipped and the CRC calculation
  * starts on the third byte.
  */
-#define ES58X_CRC_CALC_OFFSET 2
+#define ES58X_CRC_CALC_OFFSET sizeof_field(union es58x_urb_cmd, sof)
 
 /**
  * es58x_calculate_crc() - Compute the crc16 of a given URB.