hwmon: (pwm-fan) Declare pwm_fan_of_get_cooling_data static

Address the following sparse warnings.

drivers/hwmon/pwm-fan.c:176:5: warning:
	symbol 'pwm_fan_of_get_cooling_data' was not declared. Should it be static?
drivers/hwmon/pwm-fan.c:176:5: warning:
	no previous prototype for 'pwm_fan_of_get_cooling_data'

pwm_fan_of_get_cooling_data is only used in the pwm-fan driver and thus should
be declared static.

Cc: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>