hwmon: (pwm-fan) Fix build when THERMAL=m

Fix build errors when CONFIG_THERMAL=m and SENSORS_PWM_FAN=y
by restricting SENSORS_PWM_FAN to 'm' when THERMAL=m.

drivers/built-in.o: In function `pwm_fan_remove':
pwm-fan.c:(.text+0x22ba58): undefined reference to `thermal_cooling_device_unregister'
drivers/built-in.o: In function `pwm_fan_probe':
pwm-fan.c:(.text+0x22bebb): undefined reference to `thermal_of_cooling_device_register'
pwm-fan.c:(.text+0x22bf11): undefined reference to `thermal_cdev_update'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>