serial: ifx6x60: fix paging fault on spi_register_driver

[  117.240866] BUG: unable to handle kernel paging request at 815b627c
[  117.240866] IP: [<813fe94b>] spi_register_driver+0xb/0x50
...
[  117.240866] Call Trace:
[  117.240866]  [<817de977>] ifx_spi_init+0xbe/0xf0

The root cause is, spi_register_driver() is trying to write into the passed *const* struct spi_driver.

---
 drivers/tty/serial/ifx6x60.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -1331,7 +1331,7 @@ static const struct spi_device_id ifx_id
 MODULE_DEVICE_TABLE(spi, ifx_id_table);
 
 /* spi operations */
-static const struct spi_driver ifx_spi_driver = {
+static struct spi_driver ifx_spi_driver = {
 	.driver = {
 		.name = DRVNAME,
 		.pm = &ifx_spi_pm,