can: m_can: Constify struct m_can_ops

'struct m_can_ops' is not modified in these drivers.

Constifying this structure moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   4806	    520	      0	   5326	   14ce	drivers/net/can/m_can/m_can_pci.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   4862	    464	      0	   5326	   14ce	drivers/net/can/m_can/m_can_pci.o
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/all/a17b96d1be5341c11f263e1e45c9de1cb754e416.1719172843.git.christophe.jaillet@wanadoo.frSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/m_can/m_can.h

@@ -91,7 +91,7 @@ struct m_can_classdev {
 
 	ktime_t irq_timer_wait;
 
-	struct m_can_ops *ops;
+	const struct m_can_ops *ops;
 
 	int version;
 	u32 irqstatus;

drivers/net/can/m_can/m_can_pci.c

@@ -77,7 +77,7 @@ static int iomap_write_fifo(struct m_can_classdev *cdev, int offset,
 	return 0;
 }
 
-static struct m_can_ops m_can_pci_ops = {
+static const struct m_can_ops m_can_pci_ops = {
 	.read_reg = iomap_read_reg,
 	.write_reg = iomap_write_reg,
 	.write_fifo = iomap_write_fifo,

drivers/net/can/m_can/m_can_platform.c

@@ -68,7 +68,7 @@ static int iomap_write_fifo(struct m_can_classdev *cdev, int offset,
 	return 0;
 }
 
-static struct m_can_ops m_can_plat_ops = {
+static const struct m_can_ops m_can_plat_ops = {
 	.read_reg = iomap_read_reg,
 	.write_reg = iomap_write_reg,
 	.write_fifo = iomap_write_fifo,

drivers/net/can/m_can/tcan4x5x-core.c

@@ -357,7 +357,7 @@ static int tcan4x5x_get_gpios(struct m_can_classdev *cdev,
 	return 0;
 }
 
-static struct m_can_ops tcan4x5x_ops = {
+static const struct m_can_ops tcan4x5x_ops = {
 	.init = tcan4x5x_init,
 	.read_reg = tcan4x5x_read_reg,
 	.write_reg = tcan4x5x_write_reg,