hwmon: (dell-smm) Fix warning on /proc/i8k creation error

hwmon: (dell-smm) Fix warning on /proc/i8k creation error

The removal function is called regardless of whether
/proc/i8k was created successfully or not, the later
causing a WARN() on module removal.
Fix that by only registering the removal function
if /proc/i8k was created successfully.

Tested on a Inspiron 3505.

Fixes: 039ae58503f3 ("hwmon: Allow to compile dell-smm-hwmon driver without /proc/i8k")
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Acked-by: Pali Rohár <pali@kernel.org>
Link: https://lore.kernel.org/r/20211112171440.59006-1-W_Armin@gmx.de
Signed-off-by: Guenter Roeck <linux@roeck-us.net>