thermal: Replace pr_warn() with pr_warn_once() in user_space_bind()

Use pr_warn_once() instead of pr_warn() to print the user space
governor deprecation message in user_space_bind() to reduce the
kernel log noise.

Fixes: 0275c9fb ("thermal/core: Make the userspace governor deprecated")
Reported-by: Linus Torvalds <torvalds@linuxfoundation.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/thermal/gov_user_space.c

@@ -17,8 +17,8 @@
 
 static int user_space_bind(struct thermal_zone_device *tz)
 {
-	pr_warn("Userspace governor deprecated: use thermal netlink "	\
-		"notification instead\n");
+	pr_warn_once("Userspace governor deprecated: use thermal netlink " \
+		     "notification instead\n");
 
 	return 0;
 }