thunderbolt: Make tb_sw_write() take const parameter

The function does not modify the argument in any way so make it const.
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>

drivers/thunderbolt/tb.h

@@ -399,7 +399,7 @@ static inline int tb_sw_read(struct tb_switch *sw, void *buffer,
 			   length);
 }
 
-static inline int tb_sw_write(struct tb_switch *sw, void *buffer,
+static inline int tb_sw_write(struct tb_switch *sw, const void *buffer,
 			      enum tb_cfg_space space, u32 offset, u32 length)
 {
 	if (sw->is_unplugged)