hwmon: (dell-smm) Mark i8k_get_fan_nominal_speed as __init

Mark function i8k_get_fan_nominal_speed() as __init since
it is only used in code also marked as __init.

Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Reviewed-by: Pali Rohár <pali@kernel.org>
Link: https://lore.kernel.org/r/20210814143637.11922-5-W_Armin@gmx.de


Signed-off-by: Guenter Roeck <linux@roeck-us.net>