thunderbolt: Make tb_port_type() take const parameter

The function does not modify the object in any way so make the parameter
const to reflect this.

No functional changes intended.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>

drivers/thunderbolt/switch.c

@@ -459,7 +459,7 @@ static void tb_switch_nvm_remove(struct tb_switch *sw)
 
 /* port utility functions */
 
-static const char *tb_port_type(struct tb_regs_port_header *port)
+static const char *tb_port_type(const struct tb_regs_port_header *port)
 {
 	switch (port->type >> 16) {
 	case 0: