can: m_can: Count TXE FIFO getidx in the driver

The getindex simply increases by one for every iteration. There is no
need to get the current getidx every time from a register. Instead we
can just count and wrap if necessary.
Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Link: https://lore.kernel.org/all/20221206115728.1056014-5-msp@baylibre.comSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/m_can/m_can.c

@@ -1030,15 +1030,13 @@ static int m_can_echo_tx_event(struct net_device *dev)
 
 	/* Get Tx Event fifo element count */
 	txe_count = FIELD_GET(TXEFS_EFFL_MASK, m_can_txefs);
+	fgi = FIELD_GET(TXEFS_EFGI_MASK, m_can_txefs);
 
 	/* Get and process all sent elements */
 	for (i = 0; i < txe_count; i++) {
 		u32 txe, timestamp = 0;
 		int err;
 
-		/* retrieve get index */
-		fgi = FIELD_GET(TXEFS_EFGI_MASK, m_can_read(cdev, M_CAN_TXEFS));
-
 		/* get message marker, timestamp */
 		err = m_can_txe_fifo_read(cdev, fgi, 4, &txe);
 		if (err) {
@@ -1052,6 +1050,7 @@ static int m_can_echo_tx_event(struct net_device *dev)
 		/* ack txe element */
 		m_can_write(cdev, M_CAN_TXEFA, FIELD_PREP(TXEFA_EFAI_MASK,
 							  fgi));
+		fgi = (++fgi >= cdev->mcfg[MRAM_TXE].num ? 0 : fgi);
 
 		/* update stats */
 		m_can_tx_update_stats(cdev, msg_mark, timestamp);