thermal: devfreq_cooling: inline all stubs for CONFIG_DEVFREQ_THERMAL=n

When CONFIG_DEVFREQ_THERMAL is disabled all functions except
of_devfreq_cooling_register_power() were already inlined. Also inline
the last function to avoid compile errors when multiple drivers call
of_devfreq_cooling_register_power() when CONFIG_DEVFREQ_THERMAL is not
set. Compilation failed with the following message:
  multiple definition of `of_devfreq_cooling_register_power'
(which then lists all usages of of_devfreq_cooling_register_power())

Thomas Zimmermann reported this problem [0] on a kernel config with
CONFIG_DRM_LIMA={m,y}, CONFIG_DRM_PANFROST={m,y} and
CONFIG_DEVFREQ_THERMAL=n after both, the lima and panfrost drivers
gained devfreq cooling support.

[0] https://www.spinics.net/lists/dri-devel/msg252825.html

Fixes: a76caf55 ("thermal: Add devfreq cooling")
Cc: stable@vger.kernel.org
Reported-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200403205133.1101808-1-martin.blumenstingl@googlemail.com

include/linux/devfreq_cooling.h

@@ -75,7 +75,7 @@ void devfreq_cooling_unregister(struct thermal_cooling_device *dfc);
 
 #else /* !CONFIG_DEVFREQ_THERMAL */
 
-struct thermal_cooling_device *
+static inline struct thermal_cooling_device *
 of_devfreq_cooling_register_power(struct device_node *np, struct devfreq *df,
 				  struct devfreq_cooling_power *dfc_power)
 {